Correct me if I'm wrong: I think that a wound that is bleeding is a wound that's, at least in part, being protected from infection by 'internal positive pressure'.

My grandpa taught me that; to let wounds bleed freely for a while, if possible, to reduce the chance of infection.

I wonder how valid that is.

Sounds plausible. At the very least it's a mechanical cleaning. Small pieces of dirt get washed away by the liquid blood.