Masturbation is not bad. It's something that most girls are a little bit uncomfortable talking about, but you can be sure that many girls (and most of the human race!) are doing it, and so it's definitely nothing to be embarrassed by. Unless you're doing it in public places or with something unclean, which can lead to infection, masturbation should, in fact, generally be considered a good thing. Masturbation is a low-anxiety way to figure out what works for you sexually, without the worries of pleasing a partner, getting pregnant or contracting a sexually transmitted disease.

As for reasons why people masturbate, it's kind of hard to say. Everybody does things for different reasons. But I think the big reasons are it is generally safe--and it feels good!