Women masturbate for exactly the same reason as men; it’s pleasurable and can help with everything from insomnia, boredom, or good-old fashioned feelings of sexual frustration.
Percentage of women who masturbate can vacillate from just 38% to 92% depending on who you ask and how, but that really isn’t too terribly important. What is important is that masturbation is healthy and normal, however you really shouldn’t be concerned if you’re doing it twice a day or twice a month as long as you’re happy with your personal sexual experiences.
And that is one of the most important part of masturbation to focus on, as part of your personal sexuality. While we can (and do, and do recommend) masturbating with your partner, masturbation is an important part of your personal relationship with your own sensuality and pleasure. It is self-care and sex* combined, and shouldn’t be treated as less important than other aspects of your sex life.