Hey!

A function is a relation from a set of inputs to a set of possible outputs where each input is related to exactly one output.

Information systems are made up of five different functions: input, storage, processing, output and feedback loop.

In C Programming and other programmings , function is a named procedure that performs a distinct service. Functions are used because of following reasons–
a) To improve the readability of code.
b) Improves the reusability of the code, the same function can be used in any program rather than writing the same code from scratch.
c) Debugging of the code would be easier if you use functions, as errors are easy to be traced.
d) Reduces the size of the code, duplicate set of statements are replaced by function calls.

Hope it helps.

Thank you.