73 PTU
I am not too familiar with the 73, but with the 75, which cannot be so very much different. The PTU is there so that, if the L HYD (main system on the 75) is degraded, the R HYD can drive what is left of the L HYD system. If the L HYD leak is below the stand pipe, make your own arrangements. If the leak is above the stand pipe, its remains are driven, via the PTU, by the R HYD system providing power to a limited number of items (flaps, gear, nose wheel steering and hyd gen). Hope this helps!
Prober