What exactly do Accuracy Mods do when attached to Remnant Beam Weapons such as Shadow and P.A.W?

Accuracy mods reduce the minimum and maximum spread of the weapon, as well as the spread penalty per shot. While zoomed, the minimum of the PAW is 0.15 and the max is 0.25, with a 0.08 penalty per shot, which is essentially negligible. Even without zooming, it's still 0.65, 1.0, and 0.13 unless you're blind firing from behind cover. At that point your effective accuracy with beam weapons is determined far more by stability than accuracy mods. The only practical reason to equip a scope on a PAW is to zoom in on weak points so they're easier to hit, but you're generally better off with just about any other mod.