The capability to associate joints with sketch nodes is really handy for joints that would otherwise have to be offset from components by using the joint offset field in the "make joint" dialogue. Furthermore you can also use sketches with the "joint origin" command to anchor your "floating" joints, which is excellent. Furthermore, if the subject sketch is linked to a parametric component by being created in the first place "on a planar surface" of the this underlying parametric component, then when you associate a joint with a node in that sketch the location of the joint updates as you might hope it would if the parameters of the underlying component change.
In plain english: Joint offsets are very handy, but in some cases you don't want to use simple offsets in joint commands, you need something better. You can instead create a sketch and link your joint via that, and it's parametric, clean, readable, powerful.
