What is PWM Dead Band and when should I use it?
When you have an H-bridge it's really nice (as in it prevents fireworks) to have the high side turn off before the low side turns on, and vice versa. The dead time allows sufficient time for "break before make". Without dead time your bridge  (or half bridge) can experience  something called "shoot through" that is essentially a short across the rails (perhaps they're rectified mains at 400VDC through two transistors in series). At best, this causes unnecessary heating in the output stage, at worst, catastrophic failure. 

Lots more at this site.