Readability is always the defining characteristic of a good style. Separating the code over several files makes no difference to the running of the program or the memory used. The reason is twofold. Firstly, it allows the code to be broken into a number of chunks. A small file is more easily read and edited than a large one.
Global variables are more likely to be missed in a large file.
If one part of code wants changing, then perhaps only one file needs to be edited, while other files can be left untouched. Secondly, this allows code which is useful in a general context to be placed in a file for access by a number of programs. This saves duplicating code in a number of programs. The file name should be a constant string. Note that the complete contents of the file are inserted into the main program file.