NAS implies more than any router plus a USB disk has to offer.. eg incremental backup handled internally..
A NAS is faster (even the cheapest one is faster than USB2 HARD DISK plugged into a router).
It has other functionality.. sometimes lots of functionality.. whereas the airport has none..
eg .. better brands have remote access.. using several protocols, Apple airports have AFP.
Better brands have vpn, apple have none.
Better brands have FTP.. apple have none.
Better brands have media services.. apple have none.
The list is endless.. think of anything the Apple router plus USB (or even if you have internal disk on a TC it does not have it).
Apple designed it to do Time Machine backups. You can also load files on it.. good luck with backing them up once there because Apple has no provision for that.. you will need to buy a 3rd party backup software.. ie it is not recognised use of a TC or AE to store files on it.
You can use it to store some files.. but for heaven's sake back them up somewhere.
And don't use it for live files.. it is too likely to corrupt them.