Some of them can be used with Smash Bros. to be computer opponents you fight, as you do so they level up... its like tamogachi in a way. Then you can take it to a friends house and challenge their friend.
DLC exists because as inflation has devalued currency, costs for making games has gone up, but people expect to only pay $60 for a game, we should be paying around $120 if the retail price stayed pace-to-pace with inflation. Even so, a $60 game in 1995 would take at most a year to make; it now can take 5 years to make a game.
DLC, Collectors Editions, microtransactions are all tricks to get more money out of people.
I suspect once we all get too old for the market to care, you'll be left with mobile gaming, and that in and of itself will implode down to a set of staples, as mobile game development is not sustainable for long term profit. The reason I don't mention pc is because I suspect the generation of kids that grow up on mobile phones... wont view the computer as anything but something they use for work.