Functions give us the ability to divide large programs into bite-sized pieces. A computer can handle very large chunks of information without forgetting a thing. Not so people! Humans can only focus on and understand small pieces at a time. A 10,000-line program is only manageable if we break it up into many smaller chunks. Thus, we do well if we keep all of our functions, including the main function, less than 30-50 lines.
The other major use of functions is when we have a piece of code that we want to use many times. Either the code is used in many places in the same program, or it is used in many programs. Functions enable code reuse.
Functions are also an essential component of objects. There they are called methods; this will be considered more when dealing with the class construct.