Early in our marriage, I masturbated occasionally because we had PIV sex so often. As the years went by and we had kids, work hours were long, kids' got very involved in sports, etc., I masturbated even more always privately because PIV sex became less frequent - at least not as frequent as I would have liked. Why did I masturbate? To release sexual tension, to relax, to enjoy that few moments of euphroia that comes with an orgasm.
Now retired and in my late 60s, I masturbate to orgasm 2-3 times weekly. I edge every day and enjoy the build-up, seeing my precum ooze, and going through the cycle of hard-to-soft-to-hard, etc. I do most of my play in private while my wife is sleeping or is out of the house, but I do masturbate with her watching and participating. When penetrative sex isn't on the menu, she'll either give me head or a handjob or if there's a time crunch (weird since we're both completely retired!), I'll stroke while she plays with my balls and taint. So, she definitely knows that I masturbate.