The skeleton information in USD files are essentially a list of strings which are not intuitive. The Skeleton visualization extension have two functionalities
Create a hierarchical transient SkelJoint prims in the session layer
Visualize the SkelJoint prims in the viewport as a wireframe overlay on the original skeletal mesh
The skeleton visualization is toggled from the eye icon. It is under the Show By Type/Skeleton context menu and is default off. Once toggled on, a list of new SkelJoint prims shows up in the stage window and the wireframed visualization is visible in the viewport. Note that it is only designed as a visualization tool. Thus, you cannot manipulate the joints although you can select each individual joint either from the viewport or from the stage window. Keep in mind that all these SkelJoint prims are transient prims.