Everyone knows from their own lives that looking good will make it easier to get your foot in the door with the opposite sex. And if you've followed the media's fascination with the psychology of the past 15 to 20 years -- mostly the evolutionary kind -- you've probably heard about how they are given more attention even by newborns, and that they tend to earn more than their plain-looking counterparts.
But there are two even more important perks that hardly anyone mentions:
1) Should the need arise, you can fart in public without worrying -- no one around will think that the good-looking person did it. It must have been that fat ugly guy over there. (Most girls under 30 benefit from this too -- everyone assumes they never do it.)
2) You can go longer without doing laundry since your sweat not only doesn't stink, but actually smells good to the opposite sex. (See the cottage industry of "sweaty t-shirt studies.") That's a real plus during the summer. I think you have to still have a clean overall appearance, though.
The only downside to children preferring to be around good-looking adults is that they want to play with you way too much. I didn't mind that as a tutor because it meant that they'd actually behave themselves pretty well around me and not act like monsters, lest they lose my favor. But, for example, yesterday I was trying to get some reading done at Starbucks when someone's group of little kids kept hovering near me to make their stuffed animal crawl up my leg. It was cute at first, but it's hard to shoo the little boogers away when you need to get work done.