The catalogue contains all the references to the originals, all the organisation and adjustment work you've done. The xmp sidecar files do not contain everything - they miss out flags, collections, history steps, virtual copies. They have value if you ever open a raw file directly in another program, and much less so as a secondary form of backup.
1. You can delete them, but why bother?
2. LR can recreate them whenever you want by selecting the images and using Ctrl S (Cmd S if on Mac)
3. Some other programs can read them. Aperture and C1, PhotoMechanic can, but Nikon programs generally fail to do so.