Is there an explanation on why Kylo Ren wears that mask in The Force Awakens? It even has a voice synthesizer and everything.
It makes him look like a Darth Vader fanboy and really tries to be/look/sound like him.
It doesn’t look like he needs it or anything.
Psychological warfare. Vader was the death dealing wizard right hand man of the evil emperor for well over a generation. More than likely, his image is still enough to put a good portion of the population in fear. By taking up the mantle, so to speak, he is making a clear statement about the terror and power that he represents.
To put this in perspective, imagine if, in the 1970's (let alone today) someone rose to power wearing a certain uniform with a red sash around his arm with a funny symbol on it, a swept haircut, and a little mustache. His belief and intent would be instantly clear to everybody around him. He would be drawing upon the associations with the look of that person, using it as a tool. In the 1970's you would have a combination of people who survived the war as well as children born and raised in its shadow, all completely familiar with the faces of those famous people associated with it.
Just so with the time period in Force Awakens, we are a little more than a generation past from the downfall of the Emperor, and the tremors from the war continue to resonate.
Kylo Ren is obsessed with Darth Vader,
his maternal grandfather.
He tries to imitate Vader’s appearance even though there is no medical need for him to cover his face.
Additionally, the mask and helmet are worn for the intimidating appearance: They hide Kylo Ren’s facial expressions, let him appear larger and make him inapproachable to his underlings and enemies.
Helmet and mask as pieces of armor also offer protection against flying debris and would be sensible pieces of equipment for explosion heavy assault missions such as the ones seen in The Force Awakens.
Canonical answer: during Rei's dream sequence when she picks up the light sabre, we see a group of characters all wearing similar masks. (From the screenshot below it looks as if the masks are not all identical, but they are similar in appearance.)
From the context of the movie we can surmise that these are the Knights of Ren. Thus, the reason Kylo Ren wears such a mask is that he is (or was) a Knight of Ren, and the mask is part of his uniform. Of course, it might indeed be that this uniform is inspired by Darth Vader, as the other answers suggest.
There isn't any canonical explanation yet, or at least it isn't made obvious by the movie.
It is heavily hinted that Darth Vader is Kylo Ren's role model, so it's no wonder that Kylo Ren wants to look like Vader.
He meditates next to Vader's mask.
When he tries to read Rey's mind and the telepathic connection he has established backfires, causing Rey to read his feelings, we learn that he fears he will never be as strong as Vader.
But it is also heavily hinted that Kylo Ren looks more intimidating, fearsome and powerful than he actually is.
He is not a skilled Force user. First, his roster of tricks is pretty limited. He can do Force telekinesis, Force telepathy and... that's about it. No Force throw, no Force jump, no saber throw, no Force lightning. Second, he fails both times he tries to use his powers on other Force-sensitive people. When he tried to read the map out of Rey's mind, it backfired on him and caused Rey to read his mind. When he tried to pull Luke's lightsaber out of the snow, it actually went to Rey.
At the end of the movie, Snoke says that Kylo Ren must "complete his training".
Kylo Ren's lightsaber has a very rough blade (see the official movie poster for comparison of Kylo Ren's lightsaber and Anakin Skywalker's one), hinting to a less-than-stellar build quality. By the way, it is canonically established by Star Wars Clone Wars season 5, episode 7 ("A Test of Strength") that building a lightsaber is a test of Force-wielding.
The Dark Side of the force provides "abilities some consider to be unnatural" (Palpatine in Revenge of the Sith) and excessive usage of these powers permanently changes the user's appearance. Now, when Kylo Ren takes off his mask for the first time, he looks... handsome? innocent? disappointing for a Darksider? Either way, he obviously isn't yet intimate with the Dark Side.
Finally, let us not forget that Kylo Ren is one of the Knights of Ren.
The movie doesn't really explore that part of the story. We know that Kylo Ren is one of the Knights of Ren, but we are never told about any other characters in the same group. It seems that current consensus among fans is that the Knights of Ren can be seen in the Force vision that Rey had when she touched Anakin's lightsaber. This is the best image I could find. As we can see, most of these people wear masks.
I am not sure about the guy with the spear, the second from right. I think we can see his eyes, nose and some facial structure that goes around his mouth (maybe coincidentally, but not unlike Sith race). But this might be a mask after all. Wookieepedia has another screenshot that leaves little doubt that the guy with the spear wears a mask, too.
So, maybe wearing a mask is just something that Knights of Ren do.
Canon answer, courtesy of TFA Visual Dictionary page 24 ("Kylo Ren"):
Patterned after the battle gear of the Knights of Ren, Kylo Ren's helmet conceals his identity and adds to his imposing demeanor. Servomotors drive articulated arms that separate the face mask from the helmet, letting Kylo remove the black form to state down his opponents with incontained malice.
Also, on page 26 ("Dark Warrior"), the mask is called "Battered Combat Helmet".
So, there seem to be 4 "Why"s:
To conceal his identity.
This is important. TFA VD also lets us know that:
his birth name is never spoken by decree of the Supreme Leader.
To intimidate.
Because let's face it, Adam Driver just Does Not Look Intimidating. More in-universe, the existing answer by @Broklynite unpacks this angle very well.
For combat protection. It is "battered" after all
As a uniform
In the script/film, Snoke calls him "Even you, master of the Knights of Ren, have never faced such a test."
As such, wearing the gear common to the group you're a master of makes sense.
Lots of Sith wear masks. Take this quote about Darth Revan.
Without the mask he looked more human. It was easier to remember he was just a man, with all the weaknesses and vulnerabilities that implied. With the mask, however, Revan was an icon, a symbol. He was the shaper of history, an individual defined by his actions rather than his thoughts, feelings, and beliefs.
He's a mentally unstable young man looking for a direction in life. Disappointed with his father whom he sees as a foolish old man, he strives for greatnes. As it often happens, the dark side looks more glamorous. Seductive, if you will. He is trying to be a new Vader.
During the exchange on the bridge, Han says, "Take off your mask, you don't need it." Which is followed by a discussion of how Ren "destroyed" Ben Solo. The mask is an integral part of that transformation and the erasure of his old identity, along with the intimidation discussed above.
And he selected that mask and voice manipulator to be as much like Vader as possible.
I think one reason he wears the mask is to show that he is no longer Ben Solo.
I'm sure being a Vader fanboy is a big part of it, but the mask probably has a HUD and all the stuff the stormtrooper helmets have. Example, when he sees Finn stop shooting and have panic attack, he knew exactly what number he was. Same as the troopers know who each other are just by looking at the time.
On top of the reasons above, Kylo Ren sees who he is and who he used to be as two distinct people, just as Vader did. I believe the mask helps with this, which is why in the climactic scene he was asked to remove it.
