One advantage of using multiple files is that you can have functions that are private to the file (static functions). So when looking at a specific file you know it can use either functions in this specific file or functions #include-d from other files. But it cannot use the private functions of other non-included files.

This reduces the cognitive load and gives the compiler a way to enforce separation.

In short - files in C can give you namespaces -- which are sometimes a useful way to organize code.