People want to be friends with benefits for different reasons. It might be that they just want regular sex with someone that they feel comfortable with but aren't interested in that person romantically, or it might be that they do like the person in a romantic way, but for some reason a relationship might not be practical at the moment. You can love someone that you have a friends with benefits relationship with, but usually people get into a friends with benefits relationship when they aren't interested in a romantic relationship with that person.
Because sometimes people just want to f*** without any emotional attachment.
sex. they want the physical release without the emotional ones
People want to be friends with benefits because they want sex without the complication of a relationship. It makes things easier for guys/girls to get out of a situation because they aren't a couple. And it allows you freedom to date/ have sex with as many people at the same time.
And some people become FWB thinking they can turn their love interest into their bf/gf. Or change a person. Anyways, Don't do it if you can't handle the consequences.