There are three general reasons 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 data types or constants that do not exist on
the other system. When this happens, it is not enough to avoid
executing the invalid code. Its mere presence will cause the compiler
to reject the program. 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. One version might make frequent time-consuming
consistency checks on its intermediate data, or print the values of
those data for debugging, and the other not.
A conditional whose condition is always false is one way to exclude code
from the program but keep it as a sort of comment for future reference.
Simple programs that do not need system-specific logic or complex
debugging hooks generally will not need to use preprocessing
conditionals.