At its simplest, the linker just takes the set of object modules produced by the compiler and links them all together into an executable image. One of these modules must correspond to the main program unit, the other modules will correspond to procedures and to block data subprogram units.
It often happens that a number of different programs require some of the same computations to be carried out. If these calculations can be turned into procedures and linked into each program it can save a great deal of programming effort, especially in the long run. This ``building block" approach is particularly beneficial for large programs. Many organisations gradually build up collections of procedures which become an important software resource. Procedures collected in this way tend to be fairly reliable and free from bugs, if only because they have been extensively tested and de-bugged in earlier applications.