Hi
1) May be for speed of operation
2) Used mainly in ONline UPS ( for sine wave o/p)
3) Multiple DC-DC conversion
4) Most of the DSPs have builtin multiple PWM o/p modules(Dspic)
1) can be configured for driving pushpull
2) or H-bridge driving
5) Programmable dead band options is present in Dspic
Also
6) Algorithms lik PID can be easily be implemented
Refer to Triple conversion Online UPS application note from TI site