"I'm good at sex because I spent a long time training alone." So said Woody Allen, and he's right: masturbation isn't some dirty, degrading act we should be ashamed of.
Good training
90% of men and 60% of women* do it, so it must be doing our sex lives good! When they masturbate, men learn to make their erection last longer and stop themselves from coming too soon. We're not going to argue with that! Women learn to get to know their bodies better and learn how to bring themselves to orgasm. Basically, masturbation is a way of training yourself for sex à deux, even if nothing can replace the pleasure of sex with a partner.
Masturbation can also make up for discrepancies in sexual desire between a couple. When you're not both feeling the same way, masturbation can satisfy your individual sexual needs.
*Context of Sexuality study