The first reason is reusability
. Once a function is defined, it can be used over and over and over again. You can invoke the same function many times in your program, which saves you work. Imagine what programming would be like if you had to teach the computer about sines every time you needed to find the sine of an angle! You'd never get your program finished!The second reason is abstraction. In order to use a particular function you need to know the following things:
-The name of the function
-What the function does
-What arguments you must give to the function
-What kind of result the function returns
If you just want to use the function in your program, you don't have to know how it works inside! You don't have to understand anything about what goes on inside the function.
It's sort of like driving a car or using a telephone. With an automobile, you don't need to understand every detail about the engine and drive train and wheels, if all you want to do is drive the car. Similarly, with a telephone, you don't have to understand everything about the phone system in order to make a call.
The only time you need to know how a function works inside is when you need to write the function, or change how it works. It's like a car again; you need to know how a car works in order to build one or fix one. But once a function is written and working, you never need to look at its insides again.
Hope this helps,
kwrob
