I've been doing mobile DJing for around four years (while I also scratch and mix as a side-hobby when I'm not simply just producing my own music), and I've done this at least over 100 different gigs so I'm going to try and talk through experience.
While it is true that for a good portion of the show you are just pressing play when it comes to audio, you have to make up for that with performance. You don't simply go to a show, set up your equipment, put up a playlist, play it, and just stare at the audience hoping they like. You have to read the crowd in order to get a feel for what they want to listen to. And, yes, for the most part they want to hear songs they already know.
However, what this method might lack in audio-centered creativity, good mobile DJs need to make up for it in performance. Since all attention is not paid towards beat matching and mixing, most of it goes to the audience -- almost like a concert. You play a song they recognize, they freak out, you "perform" to it while they dance to it.
There really is SOME effort that goes into it. To press play and just stare at the audience while they stare at you is
the absolute worst thing any type of DJ can do.