Well you would while your backup battery powering up your ONT for about an hour.
The BBU contains a set of 4 x AA NiMH cells, so nothing to stop you swapping them out for a set of charged ones when they run out. Rinse, repeat, as often as you like. 
That's good to know, so a 4.8V (5V) supply using NiMH batteries, hmm

Does it say how much current it uses?
In theory I could have a 6V SLA Battery with its own charger and have that connected up to a 4.8 to 5V DC to DC converter and have that connected to the DC Jack Socket on the ONT bypassing the need for the BBU and that would last a lot longer than the 4 pencil batteries.
Our sub station is located at the bottom of our road and is very very old and over the last several years it has gone down a few times and has been down for several hours, so I would like the BBU to last at least 10 hours just in case.
I could plug it all in one of our UPS's we have here, that's if they still work LOL
I will have to wait and see when it finally gets done, which hopefully will be soon, because 4 years is becoming a joke now.
Paul