Achievements are like a lot of things in real life that you don't really need, but want for one reason or another, and it's usually to show off.
Some real world examples of "achievements" are:
1. Shiny wheels on your Ferrari
2. An Armani suit, or fashion in general
3. A girlfriend with massive knockers, who is you girlfriend only because she has massive knockers.
4. A sports trophy for winning the 100m dash at school
6. Hair dye
7. Penis extensions and/or breast implants, including cosmetic surgury in general.
9. Motorised automatic gates on your driveway
10. An olympic gold medal
You do not need ANY of these things to "win" at life... yet most of them are to show off, or in the case of example number 5... taste really good.
I for one, will try to get most of the achievements in a game. I'm not OCD enough to get EVERY one of them unless they happen to be reasonable to obtain. Not touching the walls in a racing game for an entire championship could just go ang get fucked srsly...
Games like GTA have had "hidden jumps" and "find the thing with the thing" for a long time now... in fact, most games had some kind of "secret" thing to find, even back in the 8bit days, and even before that! Game developers are just making more elaborate "secrets" in games, and thanks to Microsoft, everyone now calls them "achievements" which is kind of a shit word to use really.
I still prefer "secrets" or "easter eggs" or "special items" as the primary descriptive term... but that's just my opinion.