In all the Despicable Me movies, I never found a single instance of a Minion without a goggle.
- Why do they always wear goggles?
- Are the goggles merged into their skin?
The minions' character design was influenced by two sources: Oompa-Loompas and Jawas.
When they were creating the characters, Coffin and Renaud thought of other memorable sycophants of cinema — the orange-skinned Oompa-Loompas from "Willie Wonka and the Chocolate Factory," the brown-robed Jawas from "Star Wars" — and realized that short stature was a major part of their appeal.
The filmmakers ended up designing Gru's minions as subterranean, goggle-wearing mole people in coveralls who like to eat bananas and punch one another in the face. "The minions are like children," said Renaud. "They lose their focus, they're not very smart."
If Gru is "despicable", then the minions are undoubtedly "mischievous". They also engage in a lot of slapstick comedy, which means they get hurt a lot. When they do, you're meant to laugh at them, instead of sympathising and cringing. This is done by dehumanising them:
Along the same vein, goggles dehumanise the minions by partially obscuring their eyes. Eyes are very important for human empathy; studies show that children prefer peers without glasses over those with them; we also find it intimidating when people wear items such as shades, visors or eye-obscuring helmets.
On the other hand, we derive a lot of information from observing eyes by the way they are expressed. Therefore goggles serve as a compromise between dehumanising but still showing the eyes underneath, so a big range of emotions are available.
Still, just in case you were concerned about the welfare of the minions, it's comforting to know they have goggles to protect them.
They are not melted into their skin. In this trailer video for Despicable Me, a minion pulls the goggles of another one away from his head.
I always thought it was because they were low-level workers for Gru's science experiments. So they're working with caustic chemicals, manufacturing with lathes and welders, and generally going about doing a lot of work that can seriously damage your eyes if unprotected. You see this sort of thing a lot on factory production floors around the world, regardless of what's being manufactured.
Also, they look even goofier when they only need one lens to protect a single eye.