Yes. Your eye is surrounded by something called orbital bones (those hard, bony shapes around your eyes, in the skull). This in many cases would block your opponent's fist from directly making contact with the eyeball. However, you may not be so lucky in other cases, and if the fist manages to slip trough the orbital bones, your eye could absorb enough force from the punch to, for example, cause you a torn retina. This could result to bleeding, glaucoma, cataracts and lens displacement, iris disruption, macular dysfunction and retinal detachments. All which are very serious and need medical attention, lest you might go blind from the injury.
Backing up everything Ranko just said, emphasizing the detached retina part. I once did nothing more traumatic than shoot myself in the eye with a large rubber band. The opthalmologist (not optometrist) had me going back to him every two days for a week to check my vision.
Part of the issue is aharmonic stochastic resonance, like a car with back shock absorbers. Wearing glasses helps.
Glasses would break into pieces after the punch, which would be far more dangerious. It would cut the eye.
They're made for protecting your eyes. I'm sure you could find a pair made of titanium if you prefer, but just wearing glasses deters many from punching you in the nose.