Who it's for, and when
The edge of the bed is an easy position. The woman doesn't have to support her own weight or be especially flexible: if need be, he's the one who lifts her ankles onto his shoulders. And nobody's weighing on anybody, which makes it a good alternative for women who feel squashed or short of breath in missionary.
It's for couples who crave intensity and want to go looking for the G-spot together. Just searching for it can become an erotic game in its own right. It suits her: she can let go and think of nothing but her own pleasure. And him, if he likes to run the show, especially once her legs go up over his shoulders.
As for when, it's made for spur-of-the-moment desire: the edge of a bed, a low sofa or a big, firm pouffe is all it takes. But if it's the G-spot you're after, take your time. It's no magic button: not every woman feels it the same way, and it responds mostly once arousal is already running high.
Good to know: with your legs on his shoulders, penetration goes deep. At the slightest discomfort, draw your legs back or ask him to stay near the entrance. If the pain comes back every time or lingers after sex, talk to a doctor or a sex therapist.
If you're pregnant, have a fragile back or knees, or limited mobility, ask a health professional for advice first. The side-lying version (see the variations), thighs drawn up without touching the belly, has long been presented as suitable for pregnant women: again, check that with your health professional.