What are Jawas supposed to look like under their heavy clothes?
The short and easy answer: because those make for cheap, quick costumes.
The long and hard answer: Jawas look like "gaunt, rodent-like creatures, with shrunken faces and yellow eyes".
And for the robes, those are used much like the ones from Dune, to hold in moisture, hide their identity and to dissipate their body heat. The glowing eyes they seem to have is the result of polished orange gemstones that are sewn into the robes to protect the Jawas’ eyes from the bright sunlight (no mention about WHY they would evolve such delicate eyes in such a hostile environment).
The robes are also soaked in some liquid which smells awful (to us), but helps keep them hydrated, and helps them communicate (with smell).
Why wear a hood, even indoors? Why do you wear clothing over your groin, even when it's really hot? Why do women cover their chests, even when they don't cover much else? Why do Muslim women wear veils? Why do orthodox Jews wear those little hats?
Clothing is not always functional. It is also cultural, and in that regard, it ceases to require rational explanation.
Because in the original Star Wars movie the actors were children. So they needed a costume to hide that fact.
Sometimes these props come about only due to the practical, real life problems the producers face.
The robes are cultural and practical (cultural as in tradition, practical as in their doused in chemicals that allow for scent to become a factor in communication, they keep in moisture allowing for cooling, and if a jawa steals something its pretty much impossible to tell which one).
No one's 100 percent sure what they look like, but from skeletal remains it seems they are rodent-like. Also, they DON'T speak Ewok, they speak their own language called Jawaese (really, look it up) which Lucas based on sped up Zulu.
The reason for delicate eyes is likely they interrupted the evolution process, by wearing veils beneath their hoods with gemstones over the eyes, leading to no need for stronger eyes. As for Tantooine's desert appearance, Tusken raider legends claim that the ancestors of the Tuskens and the Jawas insulted some great traveler or another, who then fried the planet with a death star-esque cannon which turned the surface of the planet into glass, which then shattered into sand.
Plus two interesting facts
Sandcrawlers were left behind by a mining company which failed to make a profit, and most were salvaged by the Jawas
Jawas smell so bad that swarms of insects collect under their hoods, providing sources of food for a desperate Jawa.
Because they don't like people to see them. They obviously love garbage so they probably just wear cloaks because they like random things. They wear the cloaks to hide like camouflage as well. They probably also wear the cloaks to categorize themselves as a whole race.
They're maybe kind of relative to the tuskens, because the fact their not revealing their self's is because of their influence to the tuskens tradition that is not removing their clothing in front of someone. Because jawas like to dwell with each other (and with the droids) they don't even will remove their hood or even robe.