The idea is that you add it into the solder already in place. That in turn lowers the melting point of the mixture.

With a low melting point it takes a lot longer to re-solidify after you heat it up which means (if you do it right) you have a lot more time to work - so you can get the solder mixture for all the connections on the entire component liquid at the same time and just pull the component out in one go without damaging the board or the component.

You do then need to clean it up as its a bit messy but solder wick works fine for that.