Rocks (not so much branches) have a very high heat capacity. That means that 1 gram of rock needs a lot of heat added to raise it's temperature 1 degree. So, a warm or hot rock surface has a lot of heat in it. It also means it can give up a fair bit of heat without lowering it's temperature very much, so the rock the snake is on will stay warm for quite some time while the snake is basking. Besides, the snake can simply shift position whenever necessary.
It seems to me that a basking snake (on a rock anyway) is getting both kinds of heat. What they actually need I wouldn't presume to say.
1 adult bull snake: "Dozer"; 1.1 juvenile bull snakes: Oscar and Phoebe; 3 baby red-sided garters; 1.1 macklot's pythons