From my [own answer][1] of similar question:

> In Despicable Me 2, It is shown that minions have bad eye-sight. Without glasses, minion's vision is blurry. When because of, a purple minion, one of the minions lost his goggles, he was not able to see properly. So that appear to be reason behind wearing goggle.


  [1]: https://scifi.stackexchange.com/a/43047/6006