Syncing or Activating your EndNote account provides access to:
In addition, because it is not recommended to save your EndNote Library onto cloud storage, it gives you:
If you have previously used EndNote Online/Basic and wish to start using EndNote Desktop, syncing will get all of your existing references into your new EndNote desktop library.
Syncing with EndNote Web also allows you to share your EndNote Library, or selected reference groups, with colleagues/friends.
See:
If you wish to sync your accounts, you do this within EndNote desktop: