Achievments give you a sense of accomplishment and proof that you have done that, such as if i say i completed Saints row 2 the day it was released nobody would believe me but if i show them my achievments i can show the date
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It may be stupid to you but not to others. You should at least try to understand and at the very least respect that. I am sure there are things you enjoy doing that others would think is stupid.
There are clearly a lot of people that care about achievements or else they wouldn't be popular. So popular that infact Sony had to copy and introduce the trophy system into their own games.
Tbh, looking at them they seem stupid, but they do infact add a sense of challenge and competition to the games your playing.