i am trying to understand the logic behind autoclave and i end up with stupid question

so how it works. i know it is pressurized chamber.with controlled temperature and vacuum  inlets. thats all clear.

but what that pressure brings if the part is already in vacuum bag?
what is different than just vacuum bagged part in controlled oven?

i know it must bring something, otherwise noone would spend huge amount of money for.

so if someone cold explain me that i would appreciate.

regards
T