Each programmer works on a different file. An object oriented style can be used. Each file defines a particular type of object as a datatype and operations on that object as functions. The implementation of the object can be kept private from the rest of the program. This makes for well structured programs which are easy to maintain.
Files can contain all functions from a related group.
For Example all matrix operations. These can then be accessed like a function library. Well implemented objects or function definitions can be re-used in other programs, reducing development time. In very large programs each major function can occupy a file to itself. Any lower level functions used to implement them can be kept in the same file.