Water cooling was added to prolong the life of the turbo
Whilst the engine is running the oil supply to the turbo cools the core assembly, once the engine is stopped the turbo continues to spin winding down to rest eventually.
There is no longer any oil flow to cool the hot turbo core and the shaft and bearings will overheat as the heat from the hot exhaust side conducts is way along the shaft .Water will thermo syphon around the core and take this heat away.