Because you can't completely drain a vinyl pool. It can ruin the liner, float the whole pool or worse. I've heard mostly horror stories about draining a vinyl pool.

How would you keep the tarp on top of the existing pool water and keep the water dispersed evenly enough so that the tarp doesn't slip into the water and mix the new water with the existing water if you don't have it secured around the pool?

That would be like floating saran wrap (unsecured) on top of a bowl- half filled with water and not expecting it to slide under.

I'm having a hard time explaining it...does that make sense?