Just personal preference. Some like it, some don't. It was done in the early days, so it's "traditional", not mandatory.
Looks neat, if your engine doesn't leak oil or gas. I can honestly say, if I'd had one, it would be black.
It doesn't take any longer to clean a white firewall than it takes to clean the gloss black inner fenders, it's a nasty job but we all do it, white, black or body color, it has to be done!!
It dated back to race car days, bare aluminum or white firewall allowed you to spot leaks quickly. Starting in the late teens AAA race car drivers had to dress in white, so officials could spot blood. Bob
Airplane and boat/ship engine compartments were white for leak detection AND it is easier on your eyes. You can just see the dark colored engine components better in a bright box than a dark box.
I'd like to think as a breed, us rodders survived things with details like white firewalls so work was easier on a dark street or track pit area. Yes, that smart. My story tellin it my way.