Certain aero engines of the flat sort can have their vacuum pump damaged by turning the prop backwards.
Why would you want to turn a prop by hand?
1) It's imperative to do it to check for hydraulicing (oil in the cylinders) on a radial or inverted engine. Not to do so risks destroying the engine.
2) To check for good compressions.
3) To suck fuel into the cylinders as part of the priming process on some engines.
3) On Gipsies, to check that the impulse mag is working (it clicks - no click, no start, so you tap the mag to free it).
4) On many aeroplanes, to start the engine.
SSD