Paintball masks are mandatory in paintball games. The paintball mask covers not only your eyes but also your entire face inluding mouth, ears and nose. If you don't wear a paintball mask you can very easily become blind when a paintball plastic ammunition hits your eye.
Sweating can put moisture on a paintball mask so you have to clen it up during paintball game breaks or your line off sight may suffer significantly. Some paintball masks incluides a electrical fan to remove moisture from inside the paintball mask. This is very important when your paintball games lasts long and you have to wear paintball mask a very long time.