The Moon’s disk, even at partial phases, is intensely bright with reflected sunlight. The glare washes out most of the craters, rilles, and other surface details from view, particularly in telescopes of 6 inch aperture and larger.
Moon filters reduce the glare by only partially transmitting the reflected light. Not only will you see more surface features, but you can study them in greater comfort. These filters also reduce irradiation, which is the distortion at the boundary between light and dark areas, such as along the lunar terminator. Screws directly into the eyepiece barrel.