Prestige could affect the influence your nation has over NPCs.

Or maybe it could be the measure that determines tiers. If you increase your Prestige, you can eventually go up a tier. If it decreases, you may go down a tier. 

Those would be my suggestions.

EDIT: But you may have to rework the tier numbers if you go with my second suggestion. 'Cause Austria, for example, has more prestige than Prussia but is still a lower tier.

EDIT2: Actually, Prussia has a handful of points more than Austria. But still, the difference is insignificant so you'd still have to rework the numbers.