Why is everyone so excited about butt stuff? I think butt stuff is gross as hell.
Tighter and less lubrication so more friction. Stronger stimulation. For some people necessary (penis too small, too much lubrication comes into play, vagina too loose though that's not really a thing as far as I know) to achieve orgasm.
Also, we're partially biologically predisposed to see the ass as an extension of the vagina. Bigger butt -> more fertile, see also: those monkeys with bright red ass to signal fertile period.
And of course, especially for those who claim hypermasculinity and hate fags and shit and yo I'm tough: repressed homosexuality.
That latter one's mostly a joke
Also, for some, the part where it can be painful (it isn't always - many women
do enjoy anal sex as well if correct and enough lubrication is used) helps to get their rocks off if it's some sort of power thing.
Oh, and of course, there's people who just like butts. They're boobies closer to the vagina, after all.
And your "I think butt stuff is icky" is clearly a sign of your rampant intolerance towards homosexuals, as evidenced by the strict no-homo-policy on this board.
That last one's also a joke.