I would say that like all other human sexual responses, it's not necessarily the same from instance to instance. Humans use sex in an astonishing variety of ways, and anal sex is no exception.
Personally, I find it physically pleasurable, I find the idea of being penetrated/impregnated erotic, and sometimes I find the emotional connection with the other person to be an important component. If I were bold enough to make porn or sell my ass (someone please kick me!) I would have more different reasons to desire it. Personally, I don't find the power exchange dom/sub dynamic gives me any desire for it, but I know it does so for many.
Also, humans can learn new reasons for desire from new experiences. I recall the time when a VERY good straight (but not narrow) friend and I were having sex and he asked me to fuck him, which of course I was happy to do. He went into the experience having sex with me for the intimacy, but he said afterward that he understood (which he never had before) what the (gay) bottom gets out of anal sex.