A woman's hand has a lot more dexterity than her twat.
And, if you came recently, or are stressed out, or over fifty, or have prostate problems, or muscular back pain or other medical conditions, or are in an unfamiliar environment, or the chemistry just isn't right, or she smells, or she isn't in the mood, or she just plain turns you off, you too can experience same.
Throughout my seven years of pooning, I encountered many instances when the hand made the difference. Even now with my SO, if the timing doesn't work out, she gladly gives me a hand. There are times even when I much prefer to lie there while she attends to me. I will at other times DATY her, and roll off to sleep afterwards. In fact, I come one hell of a lot more when she uses her hand; this is because she senses the onset of the orgasm, and will vary the pitch and frequency of her touch, as well as the positioning of her hand along the shaft, thereby increasing the burst of the climax.
I'm good for every night with her, but sometimes it just work better another way...