[EricH]

Adam Savage is always dressing up as someone/something. Indiana Jones, an astronaut, a crook making a getaway... That's not necessarily historical attire, and often it isn't PPE, either.

Quite often, the others will do something similar--see the superhero special and the various ninja specials. Again, not necessarily historical or PPE.

I think they do it just because it's fun, makes a point, and/or removes the "science is not cool" stereotype.