I find this too difficult to give a fair answer. What part of the eye could be spared without any impairment to vision? I imagine no part. The lens is composed of transparent, flexible tissue, this and the cornea focus light and images on your retina. The sclera, or white part of the eye, protects the eyeball.
How can you say which part is less important? The main thing is to protect your eyes and your eyesight. This involves avoiding dusty and toxic environments where possible, and avoiding too many hours squinting at the tiny screen of a mobile phone, as well as a tablet and a television.