where are the floating power supplies used?
And in what way is it helpful in comparison with a normal grounded power supply?

 

A floating power supply is a supply that has no reference point or common point.
Where either one of it's output +or- can be attached to any part of another circuit or other power supply and not cause a short. Floating power supplys are used in bias applications where the bias voltage needs to follows a variable voltage. A floating power supply can aso be imagined as a battery. and same rules can be applied.

 

A battery in a flashlight is a floating power supply.

 

Try slogging your way thru this thread:

http://forum.allaboutcircuits.com/showthread.php?t=7805