what are friends even for?

Since I was a child, I used to think that friends were just there to serve as some sort of entertainer. I befriended people because they served a purpose for me. Is that really how connections work? So they serve you a purpose? I would befriend these people because they were funny, because I could tell them about my problems, and because I didn't want to be alone. I can't imagine myself befriending someone for the sake of just having another friend. Is this normal? Friendships have always been a reoccuring problem in my life. I am constantly confused about the nature of relationships. Mostly about what these are for. Is it the reciprocation that makes a friendship worth giving time to? When you vent, they can vent to you too. Is that how it is? Does friendship require a certain level of empathy that I don't have? I think I lack something that lets me feel the depth in a relationship. All my relationships have been superficial.