You will notice that in some cases there's current flowing through the internal diodes integrated in the mosfets's. Their spec's are not really good for some chip's: not fast enough or to high VF.
This creates additional power to be dissipated.
This current will flow through the external diodes when good and fast ones are used and this will reduce the chip's heatdissipation.
There are other reasons also but heat reduction is the most important.