I have a main PC with a 1TB drive with precious data, data that I need backups of in the event of theft etc. So I want to place a NAS some distance away from this PC. USB has severe cable length limitations. Simply using another PC on the LAN still leaves the data susceptible to theft. Ideally, the NAS will be well hidden.
How often does the data change?
It might be worth your while to make a copy to an external USB drive anyway and keep it off-site, for example at work, or ata family member's house. If the data changes daily then that is less practical unless the portion that changes is small.
A NAS in the same house is exposed to the same risks of theft, fire, lightning etc. A USB drive is easier to hide in the ceiling, and wont be plugged in and thus protected from lightning). in essence a NAS is useful if a number of PCs need to access the same data (over network or over internet if you set it up tat way) or as a media storage pool.