Robert Davis explains the NORECOVERY option when running a backup (rather than restore) command:
NORECOVERY applies only to log backups. When you run a log backup with NORECOVERY it takes the normal log backup and it also puts the database into a restoring state. This means that absolutely no transactions can run in the at database that isn’t covered by the log backup. At least not until someone recovered it. So is a scenario coming clear where that might be helpful? Think migrations.
Read on for more details. It’s not something you’ll probably do frequently, but knowing it may make a future operation smoother.