At the risk of getting lost in a swamp of not particularly good answers, it's most useful if you have scope control: If you have language keywords that allow you to designate a function as "The context/scope of this function never escapes this file," then multiple files suddenly become very useful, because as a reader you get strong guarantees enforced by a compiler, and a much easier time understanding context. The same can be said of variables and even struct fields. In very large programs it can also be useful to say, "The scope of this function/variable/etc never escapes this directory".

If everything is effectively global to begin with, you're right, it might as well all be in one file. In very large programs the lack of scope control is going to be significant problem either way.

This is where object-oriented programming yields most of its actual value - scope control.