Hermione Granger, the third of the three musketeers, was the brains of the operation. However, she ia a mudblood. This means that one of her parents was a witch or wizard and the other is a muggle. Which can easily be related to by kids from different erhnicities. She was also looked down upon for being so smart. It was claimed that she is a know it all. Well when you have the smarts, you might as well use them.
Being someone that follows the rules and studies classifies kids as a nerd, and nerds aren't cool. This is one of many reasons kids are bullied. Seeing Hermione defeat the dark lord many of times and still pass all of her classes teaches kids that it's okay to be a "nerd". By the end of the first book everyone knows Hermione and wants to become her friend. Having friends isn't all based on popularity.
For those who can relate to the isolation from being known as a nerd, reading about Hermione is like having a friend who you can relate too. A friend who is always there for you. The only thing you have to do is open the book.