If the hook end of the winch line is connected onto the the vehicle that it came from you'll have a mechanical advantage, anchor the the hook anywhere else and all you are doing is changing the direction/angle.
I've only ever used my snatch block once for doubling the load. A Ramcharger stuck overnight in a mud hole with a blown motor(ie dead pull no assist). Yanking with a strap didn't even budge him. Needed two Jeeps to anchor me.
Every other time I pulled it out was for an angle change. Its extremely handy for pulling someone back onto the trail after slipping off sideways and they need to be pulled sideways back onto the trail and you are behind them.