Flexbond has a few specific points that make it really, really handy to use for quick armor priming to get a very smooth finish without too much time or labour.

Flexbond has a high surface tension, so brush strokes even out easier than other coatings, giving a smoother surface.
Flexbond is meant to be a glue, so it 'grips' itself as you add layers - minimizing drips and runs when applied in thin layers, while still drying without a tacky finish.
Flexbond is water soluble, so if you do have drips or runs, they're easy to fix without having to repair a large area.
Flexbond dries clear fast - faster than any other brush-on primer.
Flexbond is non-toxic, and can be used in small spaces - it doesn't smell bad and won't poison you with fumes.
Flexbond, like the name suggests, remains flexible. Pieces can be flexed and struck without chipping or cracking. Toss your armor in your luggage for airport security or build designs that flex on and off without worrying about your finish or last minute touch-ups.
Flexbond will coat thermoplastics and foam with the same finish, meaning that you can coat pieces at the same time and paint them without distinct surface differences.
Flexbond can smooth Worbla's Finest Art to a smooth, paintable finish in as little as 3 coats, and Black Art in 2, though your mileage (and personal preference) may vary.