I have an idea that this causes a slight indentation in the hose once it's hot and expanded to encourage coolant flow in the correct direction. Anyone know for sure? If so, should it be near the block outlet or the heater core inlet for maximum effectiveness?

Given gravity, I'm assuming it might be more effective near the block, but to be honest, that's a complete guess.

Cheers, Jon

 

The pressure in the coolant system is what determines the direction of flow. Those plastic clamps are usually used to bundle hoses or wires together so things don't flop around.