It's likely there to make combos slightly stronger at higher levels, without making them just as strong at lower levels. Also, there's a 99% chance it's affecting the combo potency by the same value as the uncombo'd value.

Using WAR's for example, it's very likely the combo potency for Storm's eye/Path is 380 before 84, and then the uncomb'd potency becomes 120, while the combo potency rises to 400 once you get the trait. it's basically potency management across level thresholds.