As it being mentioned many times, there is almost no real point in higher difficulties other than self-challenge. Or, to be more accurate - there is no loot point in that.
Ways to change that:
- Weapon customization and better parts (laser sights, more powerful flashlights, scopes e.t.c.) are to be looted at the end of 4th and 5th difficulties.
- Create ingame encyclopedia and fulfill it with lore, arts, concepts, enemies and weapons data, W40K slang e.t.c. The most secret and unique data for this encyclopedia is being acquired upon completing 4th and 5th difficulty.
- Guaranteed Emperors gifts of purple and orange levels.