I guess it depends on game. For steam, I understand that devs can use them to properly assess how many players actually get to play and where they give up on the game. Some games do get me to achievement hunt, but others I don't bother. Anything that involves multiplayer I ignore entirely. It's kinda funny that I only feel like I care about the achivs when I see my progress in the game - while playing, I'll hardly care.

As for retroachievements, I don't bother with them. I prefer playing those old games the same ways they were played back then, without hoping to get a badge that you did some specific thing.