It's optional and helps us collect more information on what players feel was a good game, or what was a not so great one. It's very subjective but it still does help give us more insight!
Basically, if you feel good about a match after it ends because you smashed or even because you spent 90% of it in a dance-off, you can give it a good rating (or not) and if you think it was terrible for whatever reason then give it a low score (or not, again, totally optional).