BSP algorithm and depth buffer


If I have a depth testing enabled I don’t need to sort BSP tree by Z values. But is depth testing slow or something? I’ve read that BSP tree can solve the problem with depth sorting and it’s its main benefit, but what for? I’ve got the depth buffer?

thanks in advance

Depth testing is very fast but depth sorting is something else. If you have the camera looking through several colored glass plates do you have to sort the plates after the distance from the camera.
Another use of depth sorting combined with a depth buffer was to render the closest object first. This made the following tests little faster but I dont know if its a good idea on modern hardware.