I like that having a controller gives me more versatility in my seating position when gaming. At my desk, I can use either the controller or mouse and keyboard. If I get tired in that position, the long wire on my controller means I can sit elsewhere and play (my desk is positioned such that I can just rotate my monitor and play it from my sofa if I want).

I also enjoy that a controller is more portable than my mouse and keyboard for playing games on my laptop when away from home.