Okay here's my take on this FWIW:
The younger female half of this equation views herself as "more mature" than her peers and therefore wishes to be with someone older who is theoretically more mature (although I know some astoundingly infantile 30-somethings). Not only is he a more "mature" person, he will also likely have things like: a decent car, spending money for dinners out, a residence containing something other than cinderblock shelving and futon furniture. He may even be a better lover than her 20 year-old peers as he has likely had a few partners by now and hopefully knows what the hell he's doing. (Makes me thing of a bit on sex tips from one of Matt Groenig's "Hell" books....."Rember the clitoris this time, stupid!!") The money thing may or may not be a gold-digging thing depending on who's involved...more likely it's just nice to have relatively more loot than a 20 year-old so you can do things like go out for a nice dinner, trips, etc....stuff that is a "very special occasion" for the average 20 year-old guy.
The older male half of this equation I don't really get at all besides the obvious (he gets to have sex with a 20 year old woman). 20 and 30 are such different points in a person's life, I don't see how the two could get along at all. He's got a job, career, mortgage, etc and she's just worried about passing Bio 201. <shrug>
Fausto