They are for marketing but not in the obvious way. Achievements really exist to tell game developers what parts of their game people are actually playing. Sure, some obscure achievements may be very hard to get and thus not tell them anything useful, but a lot of games have support basic checkpoint “achievements” like “start the game for the first time” or “play through the first level.”

With enough of these, a game developer can tell what parts of their game were entertaining and engaging and what parts were not. Sometimes this information can be used to decide how to improve the game. Other times it may only be useful as a lesson for future games (by that developer) to learn from.