Performance: What’s the role of the backbox?

What’s the role of the backbox. Fireproofing or audio isolation?

Both, actually . .
It assures the sonic performance by giving the speaker a controlled space to work in and reduces sound transmission into adjacent rooms.
Also, the fire rated back box functions as a fire safety barrier which is recommended to have when breaking a fire-resistant wall/ceiling and in some geographical areas it is even required to have such back box installed.