I've done it for a number of reasons, sometimes because a partner asked me to, sometimes to show them something I like, sometimes because they are too tired or don't feel like having sex, or they are sick or hurt. My body can be a little fickle about orgasms sometimes so I'll take over and finish things off for myself if it's being stubborn. One of my favorite things to do is if my partner is asleep in the morning and I wake up beside them in a frisky mood, I'll start myself off and let my moans and shivers wake them so they can join in.