The bigger the hull, the more flexibility you have, period.
I have outfitted them as carriers, beam-intensive platforms, stacked tons of engines, sensors or survey modules, and support modules. Outfitted with massive sensor capacity and even just a missile rack or two, they scan serve well as a deep space reconnaissance craft that can pretty much defend itself against any unexpected pirate nests. They can also accompany colony craft into the dark and unknown frontier, offering an immediate security presence while the new colony scrambles to build power and shipbuilding facilities.
The biggest drawback of course, is that these large craft take a long time to build, so your large-hull shipbuilders need to located on your more mature worlds with ample power, population, and shipbuilding-enhancing tile improvements.