In Episode IV, why did the stormtroopers spend time making Bantha tracks and dropping gaffe sticks just to hide their killing of the Jawas?
I know that it shows off Obi-Wan's Sherlock Holmes wit but it seems like something the stormtroopers wouldn't care about.
So why did they bother?
In the movie, the whole chain of events starts with Empire troops under command of Darth Vader attacking Leia's ship, Tantive IV, and taking her prisoner.
Since the Senate still exists at that point and apparently has to be reckoned with, it's a scandal ready to erupt. A Senator has been taken by force, some of her entourage killed, and no direct evidence of her involvement with the Rebels obtained. Other Senators won't like that (what if they are next?), and they have political influence.
So some subordinate of Vader brings it to his attention, and Vader orders to cover everything up (as seen in ANH), because he doesn't want any interference or delays:
Leia is marched away down the hallway and into the smoldering hole blasted in the side of the ship. An Imperial Commander turns to Vader.
COMMANDER: Holding her is dangerous. If word of this gets out, it could generate sympathy for the Rebellion in the senate.
VADER: I have traced the Rebel spies to her. Now she is my only link to find their secret base!
COMMANDER: She'll die before she tells you anything.
VADER: Leave that to me. Send a distress signal and then inform the senate that all aboard were killed!
When Vader says that all aboard were killed, the Imperials either don't leave any living beings to tell otherwise or get Force choked. After such measures taken on Tantive IV, the landing party also has to cover up its actions. I know of no canon proof for this, but it's very likely that such orders have been given - why only hide half of the troublesome things? To anyone except few trusted by Vader, it must look like the whole skirmish has never happened. Which means, none of the people they find and question while trailing the escaped droids may talk to anyone about that. It's simply too strange to meet with stormtroopers in the middle of the Tatooine desert (where the Imperial military normally doesn't go at all), and the questions about some special droids with special data inside them don't help. Rumors can start spreading, and unwelcome questions can arise. If somebody comes looking for missing Alderaanian Princess, such a trail can be noticed and traced back.
And wouldn't it be so convenient if all those witnesses just died in an unfortunate accident involving some locals (well known to be aggressive), totally unrelated to any Empire representative, of course.
(This approach is unlikely to work on Mos Eisley scale, but stormtroopers are less strange when seen there, and they don't have to ask any detailed questions when looking for droids, so there's no need to eliminate witnesses anymore).
Without the initial events on the Tantive IV, there would unlikely be any need to cover up the murder of jawas. The stormtroopers could simply act as legitimate Imperial law enforcers. They could even kill jawas (as) if those resisted detaining and questioning, and nobody in the Empire would be held responsible.
Summary of TL;DR: because of the brute force use against Leia Organa, the political clout of the Senate could lead to something unwanted, so everything traceable to that has been covered up, with Vader's blessing, the witnesses have been killed, and those murders have also been covered up.
For review, here's how it plays out in the script:
LUKE: It looks like Sandpeople did this, all right. Look, here are Gaffi sticks, Bantha tracks. It's just...I never heard of them hitting anything this big before.
Ben is crouching in the sand studying the tracks.
BEN: They didn't. But we are meant to think they did. These tracks are side by side. Sandpeople always ride single file to hide their numbers.
LUKE: These are the same Jawas that sold us Artoo and Threepio.
BEN: And these blast points, too accurate for Sandpeople. Only Imperial stormtroopers are so precise.
LUKE: Why would Imperial troops want to slaughter Jawas?
Openly killing a sandcrawler's worth of Jawas is unnecessarily bad publicity for the Empire. As Luke's last line indicates, locals like Luke himself wouldn't understand why stormtroopers would want to slaughter Jawas who are poorly armed scavengers that don't stand a chance against military troops.
The Empire does rule through fear, but the Empire gains nothing from broadcasting their slaughter of Jawas. It's one thing for the Empire to publicly punish a rebellious senator or planet as a lesson for other potential rebels, but the Empire just paints itself as evil and tyrannical by killing a bunch of Jawas for no apparent reason (for obvious reasons the stormtroopers can't broadcast the fact that they were looking for missing Death Star plans, and even if they did reveal it the Jawas' only "crime" was crossing paths with the droids that had the plans).
The Tusken Raiders are a convenient scapegoat for the Empire. With just a little effort to fake the scene they can probably convince the locals that the Jawas were killed in just another raid by the Sand People (albeit an unusually significant target for them). As a bonus, the locals might be more willing to accept Imperial rule if the stormtroopers crack down on the unruly Tusken Raiders.
My answer is taken from some comments I made in another thread, and is a continuation of Avek's answer above. Ontop of what Avek stated, and Null makes a reference to it, another motivation may be considered as well: The Empire would be highly motivated to cover up their ongoing search for the missing Death Star plans.
As Avek mentions, covering up the arrest of Leia is useful:
to prevent outrage and more sympathy for the Rebellion is correct
they want to question Leia in secrecy about the plans and the secret Rebel base
It is also reasonable that they want to cover up the second half of the operation on Tatooine to be comprehensive about it. Killing the Jawas draws attention, even though Tatooine is a remote planet and the Empire control the Holonet news. If anyone found out about Leia's arrest, the killing of the Jawas in proximity of time and place could draw extra attention.
However, a counterpoint could be that Leia's movements were not secret (she claimed to be on a diplomatic mission to Alderaan), so even if anyone found out about her arrest, it isn't certain that they would make a connection between Leia being arrested near a Tatooine, and a bunch of Jawas being slaughtered there later on by Imperial Stormtroopers. They might, which is why the Empire would want it hushed up if anyone found out about Leia's arrest, and they certainly wouldn't want people digging around, which is the point that Avek makes.
However, this motivation is particularly meaningful when you consider that the one group of people who the Empire absolutely do not want to know about it is the one group of people that information would be meaningful towards: members of the Rebellion.
In Legends, Vader is aware that Leia has the plans and is carrying them to another location (mentioned indirectly in the Note section of my answere here). When she ditches them she knows she will be captured so she can't retrieve them, the inference being that someone else can/will, someone else who can investigate the crashed pod the same way the Stormtroopers did. Presumably the Empire doesn't want those people knowing that it is on their trail and going after the plans, especially if they are not yet aware that Leia has been intercepted. The Empire can't know that Tatooine wasn't the intended dropoff point for them.
