I think an apt comparison is touching hot stuff with a wet glove. With a dry glove, the heat has to conduct through the fabric before heating your hand. With a wet glove, the heat is conducting through the water and burning your hand. The solder, like the water in the wet glove, is a bridge for the iron to transfer heat efficiently to the pad.

I think a lot of hobby soldering guides really neglect the idea of heat transfer and thermal mass.