Tactical goggles are used to refract light and give better vision, prevent light flare, as well as prevent particles, dust, debris, and shrapnel from damaging the eyes of the wearer.
More than 70% of sensory input is done via the eyes. With only light injury to the eyes from debris a soldier can be completely or partial incapacitated.
Best case scenario your vision will be impaired by wind, sun, dust, and sweat. Worst case your eyesight will be damaged by shrapnel or debris, including friendly shells.