How does having eyes on a stalk helps crabs to see?

Answer 1

Having eye stalks assists vision by not only granting a much larger field of view when extended, but enabling alterations to the field of view as they change the stalk angle. It also creates a problem of protecting the now more vulnerable eyes when extended, which crabs handle by retracting the eyes under the carapace or simply closer to the head as a defensive measure when they come under attack by predators or other hostile crabs. They effect the extension or retraction of the eye by altering the fluid pressure in the eye stalk.

Interestingly enough, studies tend to indicate eye stalks do have other functions not related to vision, such as production of hormones responsible for growth, moulting, and reproduction.