Corrupted Flesh feeds the Cleaning Ritual, which lowers Corruption. Corruption goes up each Horde level, and if it's in the positive (IE you haven't been able to keep up with it via the Cleaning Ritual or other Corruption lowering perks) the enemies gain a bonus to their attack and defense. If it gets high enough they start getting bonus special abilities.
So, Corruption is a pacing mechanic to keep rapid-repeat prestiging from being super-viable.