What's the general purpose of a blocking bet? As far as I know, it's to get to showdown cheaply when we're OOP and to stop villain from bluffing us off of our hand. Are those the correct reasons to put out a blocking bet? And are there any other reasons I've missed?

In my last session I had AA on a two spade flop and then the third spade hit the river. should a 1/2 pot blocking bet be used here?

why can't we c/c and let villain try to bluff at it? what about when our blocking bet gets min raised?