It’s a nice image when you’re trying to affect the other actor.
How sharply can you send your point? How much bloodshed do you want to cause? In this moment of the scene are you going to prick your fellow actor, or are you going to open up a mile-wide gash? How much blood do you want to draw from moment to moment?
Of course you never want to spill real blood.
That would be unprofessional.