The convoluted design of egg crate foam mattress toppers makes them especially suitable for side sleepers. This is because side sleepers need to keep their knees, hips, and shoulders aligned for the ultimate sleep position. It's easy for these areas to come out of line during sleep either by a leg overlapping the other or the shoulder rolling in and underneath the head. If the knees, hips, and shoulders are not aligned, it can lead to joint pain.
The peaks of the egg crate mold around the body and reduce any pressure on the joints to keep the spine comfortably in place. By spreading the pressure out, there is less push on the 'greater trochanter', aka the hip bone.