Turtles are cold-blooded animals which means they can't generate heat in their bodies themselves. These turtles are sunbathing to get heat from the log as well as the sun. This is how they warm up and how they dry their shell, which prevents parasites and leeches from attaching to them. Leeches dry up and fall off in the sun.
Turtles actually need the vitamin D that they get from 12 hours of sun each day to live.