The appeal to me, in comparison to hand jobs.
It's more intimate. Pleasuring someone with your mouth vs hand is a step up, both for men and women (I'd think).
It feels different/good. It's typically warmer and wetter. Although a hand job with lube is a very nice thing too.
Visually, it's very arousing. Especially when she looks you in the eye at the same time. Creative use of a mirror or two can make it even more so for both partners, I think.
For either one, in comparison to "regular" sex, it's her doing something for specifically for me, focusing on MY pleasure. I think it's easier for the woman to control the activity, allowing her to tease and control the man's orgasm, which can add appreciably to the experience. Plus (as my lover brought up last night) it allows the woman to explore prostate massages at the same time, which is a whole other kettle of fish.
Having said that, I'd say that in every one of my relationships, I'm much more giving when it comes to oral than receiving. Even if it starts off as focusing on me, I think I can count on the fingers of one hand how many times (in the last 20+ years of being sexually active) I got mine and my partner didn't get hers. And I have no issues with that!

I don't want one "all the time" and to be honest, I often don't orgasm from them. But they're a great warm up for the main event.
C