The Assembly Hall is where the Avengers team sometimes meets up to do research on targets or prepare for missions about to take place. This is also where most of the teams gear is kept. Such as the outfits that they wear during combat, and things like that. This is where you can find those things. You can also find the suits that Iron Man has options of wearing in combat. Some other members of the Avengers can wear these suits as well, but not many can get by JARVIS to do so.