Actually, you never need the ESU utility
That might just depend on the firmware version and the "locale" the DNS-323 is being used in - so - whilst an English speaker in the United States, may never need the ESU, the same might not hold true for everyone else.
I have not looked at the recent versions of the ESU but in the early versions it was the only way to set - let's call it a "code page" (remember those ?) that would determine which non english languages would be supported. I'm not certain of the exact term, and I'm too lazy to load the ESU to find out, because I, like you, don't need or use it.
The need for this to be done may have disappeared in the 1.04 upgrade - when unicode support was added to Samba.
The ESU was also the method used to install the BitTorrent client.