Complex sentence: Most computer applications are distributed in a form that includes executable files, but not their source code, which is useful to a user, programmer, or system administrator who wishes to modify the program or understand how it works.
Split into simpler sentences: Most computer applications are distributed in a form that includes executable files, but not their source code. If the source code WAS included it would be useful to a user, programmer, or system administrator who may wish to modify the program or understand how it works.