The design is relatively similar to the age of sail's "gunboat" concept. A relatively small ship, with a single large forward-facing cannon. They filled the role taken by the torpedo boat and then the Destroyer in more modern times - a cheap vessel with little to no defenses, that could take down larger vessels using swarm tactics.
The difference here is, of course, the absurd size. That cannon isn't cheap or easily replaceable. I assume it's more of the naval equivalent of a siege weapon, intended to conduct shore bombardment, force its opponents to come into range of the rest of the fleet, and/or to break an exceptionally heavily armored target at closer range.
EDIT: The other advantage of a gunboat is that the small size and shallow draft let it operate better in coastal waters, rivers, lakes etc while still having more punch than a swivel gun. That's out the window here too.