>I understand the definition of PD but WHY is it used?
As stated earlier, if plenty of airspace to work with the controller can issue a P.D. clearance when workload and radio traffic volumn permit.
>When is it used?
Most often sometime prior to normal top of descent point for a lower altitude crossing restriction.
>How often is it used?
IMHO, I'd guess as often as traffic and workload permit. Removes burden from ATC and places it upon pilot --who probably wants it so he can perform descent the way he wants rather than the way ATC wants.
Unless traffic volumn is real high, I see this just about every flight up/down west coast. Usually ATC issues PD clearance shortly after we "request lower."