It's a little tricky. Ideally updateWorldTransform (UWT) doesn't change state or have side effects. The work needed for physics is related though (it is a constraint like the others), so it makes sense to do it as part of UWT, even though it is stateful. We needed a way to control when the state changes, so we added the Physics enum.
For example, you may want to use UWT, manipulate some bones, then UWT again. If physics was advanced in both UWT, it would advance twice within one frame. If you're doing that every frame, now your physics are moving twice as fast as they should! With the enum, you can choose to not advance physics on one of the calls.
