Generally there are three kinds of reason to use a conditional.
A program may need to use different code depending on the machine or
operating system it is to run on. In some cases the code for one
operating system may be erroneous on another operating system; for
example, it might refer to library routines that do not exist on the
other system. When this happens, it is not enough to avoid executing
the invalid code: merely having it in the program makes it impossible
to link the program and run it. With a preprocessing conditional, the
offending code can be effectively excised from the program when it is
not valid.
You may want to be able to compile the same source file into two
different programs. Sometimes the difference between the programs is
that one makes frequent time-consuming consistency checks on its
intermediate data, or prints the values of those data for debugging,
while the other does not.
A conditional whose condition is always false is a good way to exclude
code from the program but keep it as a sort of comment for future
reference.
Most simple programs that are intended to run on only one machine will
not need to use preprocessing conditionals.
The C Preprocessor