Why to have DLC's

[Raw Kryptonite]

Flight sims have DLC, but they're called mods since they're (usually) done by fans, not the professionals who made the game. DLC is a mod pack that's put together by professionals who often need money to justify bringing it out, unless they think it will stimulate sales or maintain their customer base. Whatever the reason.

MS Flight Sim games and others don't survive for years based on the out of the box game. Most would die off quickly just like anything else if that was the case. Mods keep them alive and more up to date.
Also, players import planes from one generation Flight Sim (at least with MS back in the day) to put in the other one, so you've paid for another game, not much different than buying DLC. Then there are the expansion packs---that's the same as DLC, but you get the media.
DLC is nothing new.