Nowadays teens have lots of stuff to do - they must go to school, do their tasks, help with the housework and their younger siblings. Also, they sometimes have arguments with their parents and have problems at school, so, they really need somebody to talk and tell about all the troubles and just to have a pleasant time.
Friends are sometimes really helpfull when a teen has problems or arguments at home - the point is parents' mind works in different way than his, so it often leads to quarrels, but when talking with a person the same age as you, the chance to understand each other grows. So, in the atmosphere of friendship it can be easier to find support which we all need so much.
As I said, time spent with friends is a great pleasure for a teen. He feels that he's accepted and listened and he is important in this company. In a great one he can find true attention and confession which are also really important and even glad him.
Alongside these facts, spending time with friends also helps the social skills development - a teen learns how to communicate, what to say or reply, how to start a discussion in different situations, and this, I can't miss that, will play a great role in his life as an adult already.
That's true deal with friends who are the same age as you sometimes can be difficult and painful - teenagers also quarrel and even so seriously that they start to think of each other as enemies. But still, support of good relationships worth it.