Congestion Penalty

I see all airports on the game have a congestion penalty percentage. 

However they are all at 0% and i have never seen them actually movie from that, whats the purpose of them? 

Congested airports will decrease your on-time performance. An airport's congestion penalty should register above 0% if the number of daily flights (using leased gates + airline-built termminals) exceeds the airport's original slot capacity (without airline-built terminals).

Never actually seen this happen...