Writing large programs effectively requires decomposition of the code into several independent modules. This makes the program easier to maintain and edit. This is done by taking the problem and breaking it into small, managable pieces. A function is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code. This helps in decomposition of the code into smaller independent modules. The task performed by a function can be summarised as taking as input a set of variables and returning a value after doing computation with these values. The value of the input variables may also be updated during the computation. Since the functions are written independent of the main code, the same function can be called from the main program with different input variables. The allows reuse of the code and hence shortening of the code.
An example of a function, say you are making a program that calculates sales tax and returns the total payable amount. The function would ask for a subtotal(s_total) and the tax percentage(p) as arguments, then take that s_total and multiply it by p/100 to calculate the sales tax(s_tax). After this, the function would calculate the total payable amount by adding sales tax(s_tax) and sub total(s_total) and return it to the main program. This function can be called many times from the main program for different customers by proving thier sub total and sales tax to be applied.