For me, it's because its a mobile game. I can play it when I can't play other things. This game doesn't do anything unique but be Star Wars. Maybe that'll change with future content patches!
Open the app, spend my energy by using sim tickets, noticed I didn't get anything, and move on. I easily get the 600 guild tokens and rank around 50-100 in Arena without much effort (expected to change with time). It's pretty great for loading screens when I play larger games, too!
So! The fun comes from it being so casual and mobile. And, whenever I open my roster it's kind of fun to think that maybe one day I'll have a good squad that I'm happy with. I also like to see new characters and scrolling through the forums. I think I read the forums more than I play the game...