Obviously helpful when one is between partners, "masturbation can be fun," as one of the characters in the musical Hair opined. But its usefulness isn't limited to life but extends to erotic romance.
Unless you're writing about a hookup (and why would one bother? They have a boring sameness unless someone gets killed) having a character masturbate is a good way of getting a sex scene into a novel early, if one is tracing an entire romance from "cute first meet" until "happily ever after."
I've done this several times. A masturbation scene is really useful in historical romances. In the past, loose conduct had penalties we don't worry about anymore--unwanted pregnancy, disease, death by stoning and so on. Heck, the "death by stoning" outcome still happens in some unfortunate parts of the world. So masturbation is really handy.
So I wrote the below in Desire in Tartan, set in 18th century Scotland:
She crawled back into her bed, grateful for the lingering warmth of her hot water bottle, which echoed another warmth…between her thighs. She squirmed, but wondered why she felt self-conscious, with no one around to witness what she did.
With a deep breath, she set her fingers upon her woman’s flesh and felt an answering throb. She caressed, and the throb increased. Warm liquid oozed onto her hand, and she rubbed that in the same way she spread lotion onto her cheeks.
The heat increased, and she became acutely conscious of the soft, worn linen of her old chemise against her skin. She cupped a breast while she played with herself.
Desire in Tartan is at http://bit.ly/DesireTartan
And in a futuristic I wrote set on the moon a couple of centuries in the future, my heroine is dreaming about a lost love:
A moan escaped her lips as she palmed her breasts and imitated her fantasy lover. Her hard nipples peaked against the thin film of her Slicksuit. Desire raced to her core and her clit twitched with need.
Tension gripped every muscle, and she looked around to make sure her car was still empty before opening her Slicksuit’s crotch. She sought her pussy with needy fingers, first dipping a finger into her slit, delving for cream, then spreading it on her clit. Wanton need whipped through her body and her heartbeat sped as she imagined Gideon making love to her.
She thrust a finger into her pussy and moaned. Fire scorched her nerve endings as she fantasized about making Gideon come. She curved her finger, seeking her G-spot, and rubbed it while caressing her clit with her thumb.
If you like what you read, find "The Moon Maiden's Mate" in Naughty Chances, an anthology that was released December 1, 2015. http://tinyurl.com/NaughtyChances