Same thing, basically. Other than tricks that require a well-spinning tama (clutches for example), they mostly just help prevent the string from twisting and affecting the Tama's movement/rotation.
I think it's worth it, for me it helped a lot when I was learning lunars etc, the control is much better for the initial spin movement with tricks like that.
I've noticed with the bearing a quick tug of the tama from the ken releases enough tension from the twisting. This was especially helpful when learning lighthouse. When using a regular bead (varied from bead to bead and style/quality of string) the ken would not sit still before the pull up.
I was wondering what the sizes were on strings and bearings for kendamas. I was looking for some bearings on like eBay and Amazon, but I just want to get a bunch in bulk, as well as a roll of string. I saw some people used 1mm string, but I didn’t know how accurate that was.