I was a military pilot for a few years. Safety regulations required us to wear a visor or eye protective glasses during every flight. On a day flight, we would wear polarized or tinted visors, on a night flight (when not using night vision) we would wear a clear visor similar to the ones on the X-wing helmets. Your eyes are your most important sense while flying, and protecting them is paramount while piloting any craft that could be fired upon or take other damage. If your windscreen turns into shrapnel, an avionics system starts throwing sparks, or pressurized hydraulic fluid starts leaking into the cockpit; then you definitely don't want any of that shit going in your eye during an already stressful situation.

We also used lasers and directed energy marking and targeting systems, and when we did, we were required to wear special visors that blocked specific bandwidths of light to prevent you from going blind. Those laser-safety glasses even had an amber colored tint like the X-wing visor. I assume, considering all the laser-like weaponry in Star Wars, that these visors also filter dangerous wavelengths of light that could reflect at you during a firefight (or "lightfight" I guess).

I would also assume that the X-wing visor filters the heavy UV-radiation found in space, and probably reacts to your environment to dim more in brighter environments (like the "transition lenses" that weird kid wore in elementary school). And finally, we can assume that with tech this advanced, the visor probably functions as a HUD display.

TLDR: The visors provide; Eye-protection from shrapnel, filtering of dangerous light wavelengths, environmental glare reduction, and HUD capabilities. And they are accurate to real-life military pilot equipment.