Humans are social animals by nature, so we build networks of people we associate with because it is what humans do. Friends are also a support network outside of the family that we often have things in common with, so it both validates our identities and reinforces them. This is important for everything from self confidence to dating to working and going to school. Also, friends help to socialize us into society, and help teach us what is acceptable behavior.
Most of the other posts mentioned that human beings are a social animal, in need of social contact. Friends serve as companions and as sounding boards when you need to get things off your chest or when problems arise that need to be discussed. I have always treasured the friends I made in my youth, and I can tell you that old friends are the best. I always feel comfortable around old friends, who already know your past and accept you for what you are, and they are the first to come to your assistance when needed.
As #4 points out, we are biologically social creatures. There are dozens of scientific studies that show that people who regularly interact with several friends live longer and much healthier lives. The exact mechanism by which this works is still the subject of debate, but we do know that people who are socially isolated are more likely to become depressed, are more prone to heart disease, and have weaker immune systems.
Friends are valuable because they can share common interests and experiences with each other. This means celebrating happy times and occasions, enhancing the enjoyment by adding their pleasure to yours. This also means being present to help and support you during difficult or hard times when you need help or protection. True friends can pick up their relationship with each other even if they have been apart for long periods of time because they share that common history - they already know how the other person thinks and reacts, so they can move directly to sharing in the moment with minimal time spent in getting acquainted or setting expectations.
A true friend is valuable because they can offer a person a lot of support (emotional, financial, etc) for the many trials in life that are certain to come. A true friend is also someone who has their friend's best interests at hand, can offer different perspectives on situations, can uplift their friend when they are feeling down, and can be someone to share the positives in life with. We could all benefit from having true friends.
Friends are a risk. When we make friends, we become vunerable because we allow others to see our flaws and we let down our guard in front of them. This is a great practice for humans because in order to find success in all phases of life, risk is involved. Being friends with others means we have practice at dealing with risk.
The most important reasons friends are valuable is that humans are social animals. In other words, we really do need people to talk to and be around. People who are loners, or alone, are not usually the happiest sort. Isolation is just not good for our health. We all need someone to tell our problems to, to hang out with, and to enjoy spending time with!
From the perspective of the social sciences, friends are valuable to us because of the support that they can give us. This support can take many forms. Some of them include:
- Emotional support. When we have troubles, friends can be emotionally important because they can make us feel as if someone cares about us. This can lessen the sense of stress that we would feel in such situations.
- Cognitive support. In such times, they might be able to help us figure out what to do.
- Tangible support. When we have troubles (or even when we just need a little favor), friends can help us out by doing things for us.
Looked at in these ways, friends are important to us in times of trouble because they can help us figure out what to do, help us do things, and/or simply help us to feel better.