Another reason to use specific include files rather than one big file is the way interpreted code works on the web server. When a php file is first loaded by the server, the entire script is "pre-processed" by the interpreter which builds a global list of functions, variables, etc so the code can execute properly. Only after this entire file scan completes does the code start executing.
The time to load the file from the filesystem is much less than the time to do the initial file scan.
As far as I know, anyway