Social interaction teaches a child a great deal about herself. We can help her gain skills in initiating a friendship - which simply involves being able to greet someone and start a conversation. There are then the more complicated skills of maintaining your own identity while you focus on what you have in common with a friend, and resisting peer pressure. When a child has trouble with these, parents really have to work at helping a child express her own opinions, even when they differ from her friend's, and refuse a friend's unreasonable requests. The skills in empathy and impulse control have to be put to use in this context, too.
Children benefit enormously from learning how to make and keep friends.
Popularity isn't easy to predict. It's fickle - a child is popular in one class, or in one school year, and not in another. And I don't think popularity is at all necessary to a child's self esteem. Children are popular for all sorts of reasons - many of them superficial. I'd worry about a child who craved popularity.