Why turn propeller by hand on old planes before starting engines?

Turning the propeller by hand on old planes before starting the engines helps to ensure that the engine's oil is circulated and that the pistons are lubricated. This practice can also help to clear any potential obstructions in the cylinders and verify that the engine is free to rotate. Additionally, it allows the pilot to check for any unusual resistance or sounds, which could indicate mechanical issues before starting the engine.