Battery Backup Unit on New Xfi WiFi Router

We had a power failure a few weeks back and it appeared  that the battery backup unit that was delivered with the new router kicked in and kept out wifi running for several hours.
Last night we had another outtage, preceded this time by a huge power surge that you could actually hear. Power went down with a bang, and the battery backup unit went stone cold dead. No indication of life, and the wifi in the house was gone (the light on the router did show yellow for a while). When the power came back on some 6 hours later, so did the wifi, and the battery backup unit showd signs of life (green light flashing every second or so).

Today I tried to contact Comast to ask about the apparent failure of the battery backup. What's the point of a battery backup that fails when the power goes out???
Ended up that the only way to actually interact with a human was through a chat session with someone in India. I was eventually told that the battery backup was ONLY FOR PHONES (landline?). I didn't think that was the case, which strarted a long circular stream of unhelpful info until I disconnected.
So, can anyone say just WHAT the battery backup unit is supposed to back up? I don't recall anything in the package that said anything about it, other than how to install it. Does it actually NOT provide power to keep the wifi running?
Maybe this last power failure was such that any comcast internet data was not getting to our pipe.

But it seems odd to have a battery backup unit that doesn't.