Its for the gradual procession of larger and larger ships. After the Gigas goes down at the hands of the Icanus, something needs to take down the Icanus.
Other than that, I guess the Gargant would be awesome for one of those Death Star Trench Run type missions where you're basically dogfighting on the hull of a large ship. As in, you have to hug the hull close, or get shredded by turrets, and get to a certain area for something...(infiltration, sabotage)? It would be a nice mission, and something that doesn't seem to have been done before (in FS at least). There doesn't even need to be allied capships attacking it.