Elaborating on what Krile said:
Suppression is a +10% hit chance for affliction spells. This means drains, curses, Fears, Death Coil, Corruption, Seed of Corruption, Siphon Life and Unstable Affliction. It does not add any +hit to Demonology or Destruction spells - the most important of which are Immolate, Shadow Bolt and Soulshatter.
All mobs have an innate 1% chance to resist spells. As a mob is considered a level 73 (or rather, 3 levels above level cap) for all intents and purposes, at level 70, they have a 17% chance to resist spells. Since 1% of their resist chance cannot be avoided, you effectively need to stack 16% - or 202 - spell hit in order to make the most out of your DPS. After you come closer to the hit cap, Suppression ultimately loses its value, and you are able to start subbing in points elsewhere (the Destruction tree, namely).