What are those strange mechanical devices that drop on the back of boats?

[Blanco]

I've noticed on a few fishing shows that they have these things that drop into the water on each side of the out board into the water, but they never say what they are & why the have them?

I'm assuming they're used in shallow water to anchor the boat?
But if so why not just use an anchor?
Seems it would be cheaper..?
I just saw on the Bill Dance show him using one on a john boat.....?
I looked on google for pics of what I'm asking about but...... Being I dont know what they're called or do, had not luck.