I have mentioned this elsewhere, but here goes again....
Standard turbos have a pressure fed plain bush bearing. When you stop driving the turbo can take a while to slow down and if you pull into the driveway and cut the ignition, the turbo will continue to spin for a short time, albeit without any oil pressure. If you have been sensible and driven nicely for the last few minutes, and you have a good quality oil in the car, no biggie, the oil present in the turbo will protect the bearing until it stops spinning.
If the oil is old / crappy grade and/or you have been fanging it home (or anywhere) and you shut off straight away either the oil in the bearing will drain away leaving the bearing to cop a hiding as long as the turbine is spinning, or the oil will stay in the bearing and get cooked, stuffing the engine oil AND your turbo!
I gotta say, I am getting pretty sick of hearing about how turbo timers let the turbo "cool down". Nothing will cool down by letting your car continue to idle.
They will, however, extend the life of your turbo when used properly. I just stay right off boost for the last 3-5 minutes of every drive and use a 30 sec timer on the TT. This makes it a lot easier when getting fuel etc as you don't end up standing around waiting for the car to shut off before you can refuel.