A simple question that in appearance evokes a rather simple answer, but (of course there will be a ‘but’) it seems not to be so trivial if we take under consideration different scholars’ opinion. For example, a Hegelianist will answer that friends are for reaffirming our being by recognizing our self-consciousness. A Dawkinsist will affirm that friends are for interaction in order to create an established social structure that will boost the odds for my genes to survive. A Jehovah Witness will probably say that they are the creation of God (which can say everything or absolutely nothing, depending on your views).
So, different perspectives that can easily be intertwined or on the contrary, be completely distinct from one another. Of course, these are only a small taste of the infinite possibilities. A New Age hippie might answer that friends are to help spread the endless love we and the world have to offer. A hard working broker might say something cited from somewhere but really think that a friend is to create a competition and make one-self improve skills and dare. A soldier might say that a friend is to save your life. To that last perspective, a gangster might also join as at times it seems somehow similar.
So what are friends for? The simple answer will be all of the above, each person depending on the perspective and the social role she procures to follow. There are, however, a few dilemmas that create difficulties to some and understanding to others. They are best introduced in social psychology studies as social experiments (here are about laughter and Q&A). In them we can perceive and understand the difference in the examples above. One illustration is how we divide our social network into different scales of friends when the most common will be a social configuration of about 150 people, from family (first of kin and close relatives) to friends (best, recent best, acquaintances etc.). This is why for a common person there are about 150 facebook friends that he actually know relatively well (out of which 4-8 he will chat on a more regular basis) and the rest that are for filling/feeling numbers.
Today, with the Internet networking some even claim that we passed from 6 to 4.74 degrees of separation (read here). Even though, I tend to agree that, in spite of the common saying, we are not becoming ridiculously international (nice TED talk about it here).
Be the need for social interaction as it may, one thing is for sure, friendship is necessary. If we learn anything from the different studies, as for example the prisoners’ dilemma is that we can choose the relationship we maintain with our friends (confessing and sharing in the plea dilemma or sharing equal gain in the negotiator’s dilemma). The reiteration in the interface of the dilemma is about the interaction. I mean, if you choose to be the individual that wins the most, you have to take the risk to lose the most etc.
Taking under consideration this delicate and rather instinctive self-consciousness I tend to apply trispectivism to whenever I´m facing a conscious decision. Where I try to figure the best possible outcome reasoning the individual (me) with the universal (friend or group of friends) and the probable interaction between us. For some of course it may sound somewhat rational and cold but if it will prevent 80% of misunderstanding and quarrels, for me, the preference is simple. As to how, well, relating to your natural self interest alongside with our dependence on the Other, reciprocal trust by association is probably one of the best methods, wouldn´t you agree?