Social comparison theory was derived because humans are highly social species. The theory is that we seek to evaluate our abilities by comparing them with those of others. This behavior helps us to understand ourselves better. Social comparison happens in two different ways. First, upward social comparison is that we compare ourselves with people who do something better than us. Second, downward social comparison is that we compare ourselves with people who do something worse than us. Both can boost our self-concepts.
This really happens in our lives. In my experience, I was always curious to know my rank in my class after every exam when I was in high school. I don't really care my exam score itself and how I feel about it without any comparison, because the only important thing to me at that time was how much I did well compared to others. Also, in fact, I want to know whether my friends and others perform well or not, and how much they perform well. I think this is caused by social comparison theory which is fairly pervasive throughout many people. If I could know how others perform, then I could feel better because I find that I do better than others, and sometimes I could realize that I have to do better and cheer up. Also, by knowing someone who performs similar to me, I could be relieved and feel comfortable because I know there are people who do similar to me.