Who it's for, and when
It's one of the three great basic positions, along with missionary and doggy style, and it suits almost every body type. It even smooths out height differences. The one thing to watch: if she's noticeably heavier than him, she keeps some of her weight on her knees and hands so as not to squash him.
It works for beginners as well as couples who know each other inside out. For the woman who wants to find, on her own terms, the rhythm, depth and angle that feel good. For the man who loves watching his partner, caressing her breasts and, for once, lying back and letting it happen. And for anyone after penetration that strokes the clitoris too.
As for timing: for her to take the lead partway through lovemaking, or on nights when he'd rather go along for the ride. It also works for winding down gently after a more athletic position, when you're both starting to tire.
Precautions: be generous with a water-based lube (see step 1). If deep penetration hurts, stay shallow, and if the pain persists, talk to a doctor or a sex therapist. If you're pregnant, have painful knees or a painful back, or limited mobility, ask a health professional for advice first (lying on him with your legs straight at least spares you from kneeling).