@Roger-Needham-0, Both is accurate. Some people like to enable offline Navigation, to reduce all possible intervention from things like traffic info and alike. Also some users mention that automatic on/offline switching comes with some quirks.
Concerning the auto sync feature: this is not part of Navigation. You can have offline Navigation enabled, while syncing your routes over Cell or wifi network. You can also completely disable cell networking, and sync routes on your home or hotel wifi. So having enabled offline navigation does not limit the syncing of routes in any way.