The Mirror of Erised

So in the beginning of this chapter it is the start of Christmas time, the snow has just fallen, and people are having fun. Ron tries to attack Malfoy for insulting his family. But Snape somehow appears (like he usually always does), and takes points away for it. Something I've noticed is that their point system in the book seems to be unfair. For example when the troll incident happened McGonagall herself took points away from Gryffindor, but Snape though you haven't seen him take any points away from Hufflepuff and Ravenclaw, I can almost guarantee he takes just as much points away from them, that he does Gryffindor. Also that he probably has never taken any points away from Slytherin. Unfair, I think so. Hermoine goes off for the holiday's and Ron stays because his parents were going off to Egypt to visit their son Charlie (or Ron's oldest brother). Harry learns Wizard's chess (just moving pieces compared to normal chess), and also that you can't go into the restricted section of the library without a referral from a teacher. So later in the chapter Christmas hits and Harry has a bunch of presents waiting for him in front of his desk. So Harry gets a maroon sweater from Ron's mom, and also an invisibility cloak from an anonymous person. All the note said was "use it well" and that it belonged to his father. So throughout the whole day Harry kept thinking about this one thing. That night he figured out that he can go anywhere throughout the whole school and nobody would be able to see him. So believe it or not he goes out. Pulls a screaming book in the restricted section trying to search for something about Nicholas Flamel, runs from Filch, and Snape, and finds this room. It looks like an unused classroom, and his attention goes to this mirror kind of out of the way. He looks in and sees people that at first he doesn't recognize. His family. His parents behind him and his grandparents also (there's more people but it doesn't say who they are so i'm not going to assume that I know). But he goes to bed and goes back the next night with Ron. Ron though sees himself as Head Boy, the captain of the Quidditch team, holding the house cup, and holding the Quidditch cup. So the next night Harry goes by himself. He's starting to get mezmorized by the fact that his family is here. He goes in and only after a few moments of gazing into the mirror. Dumbledore talks to Harry. He tells him what the Mirror does. That people have gone mad gazing into the mirror, and that he should not go looking for it since it is leaving the next day. So my question about this last part is, what does the mirror of erised have to do with the story line? Is it going into more detail about what's going on in Harry's head. Or did she simply want to include another chapter? I have tried to look up these questions on google. But so far I have not found review on why J.K. Rowling wrote some of the parts. If anybody out there does know. I'd very much like for someone to leave me a comment about it.