Question: When this movie was first released, how did everyone know why Darth Vader has to wear his armor? It's not explained on screen.
Answer: Obi-Wan describes him as "more machine now than man," which makes it clear a lot of his body has been damaged. The novelisation of Return of the Jedi apparently references him falling into a "molten pit" and getting burned.
Answer: Back in 1977? I don't remember anyone even caring that he was in the armour any more than people wondered about the stormtrooper armour. It wasn't until ESB that it became apparent it was a life support thing more than just for looking menacing.
kayelbe