For men, the appeal is often just that it’s taboo, or possibly feeds into their fantasy of degrading their partner to some extent. The fact that it’s “dirty” is what makes it a hot visual, but assholes usually don’t feel better than pussy.
For women it seems like it very much varies by the person whether or not they find any enjoyment at all in it, but some like it for the same reasons as men (mostly getting off on the simple idea of something up their ass), while for others it’s a distinctly different physical sensation that they may find enjoyable or even preferable. One girl I know says she likes it because it feels like the penetration is deeper, and while having sex with her vagina feels rather isolated (i.e. the physical sensation is mostly in the one canal built for that purpose), anal feels more like her partner is taking control of her whole body and is more intense, intimate, and appealing to her submission in that way.