The idea is to prevent the brake from self energizing, which is what happens if the rear of the pad has equal or greater contact pressure than the front (roughly speaking). If the brake is self energizing, the contact pressure is no longer fully under your control, due to twisting of the caliper arm causing the contact pressure at the rear of the pad to increase, causing further increase in braking force (that is the self energizing part), until the caliper deflects so much that the braking force finally reduces. The cycle repeats at a frequency determined by the system properties, which is typically in the audible range.