The design is already there since the 70s'. Its because they want to desperse the exhaust gases below. A good example would be those old Mercedes Benz.
If u notice cars with straight and moded upwards type, their bumpers get blacked out by carbon, & unburnt fuel. A mess to clean. Also heat rises. In this case the hot air carries the mentioned dirt above. Which some manufacturers claim contaminate air more easier.
For stock cars with straight exhaust tips, they are usually smaller diameter than the whole pining to maintain an exhaust gat jet that fires of in a straight line. and the exit point is sometimes longer off from the bumper.
A silple test can confirm this. U know how a cold morning when you start ur car and some water vapour come out. Notice that it goes up. If you have a VIOS or some car with bottom faced exit pipe, look at how the exhaust gas is dispered by the floor. It makes it easier to understand why.
LUKA also has a point in saying that the design is to accomodate the newer large bumpers for a more flusher or hidden exhaust look. Also the design does not offer any improvement in performence either.
Hope this helps.
