Don't be too derisive here. Aircraft and crew utilisation is a big part of profitably. There's a reason that airlines are always trying to speed up boarding and lower turnover time, every minute counts. When DL does something like this, it's very expensive.

Let's say there a route that is frequently late and passengers are unhappy about missing appointments and stressing about tight connections. DL spends a lot of money to increase the duration by 30 minutes, that results in happier and less stressed passengers. That means DL is spending money to make customers happier and less stressed. It's not such a terrible thing