Think of these saddlebag-style panniers as soft-sided milk crates that hang on your bike's rear rack. You can leave them on the bike (and lock them on using the anti-theft ring) and put your stuff in as needed, be it grocery bags, a briefcase, a backpack, purse, lock, picnic blanket, jacket, typewriter (hey, do your thing!), or any other object that can easily fit into a 15.75″ wide x 7″ deep x 12.6″ tall, flexible rectangular space.
Clarijs (pronounced Claire-ice) makes these panniers out of a durable truck-tarp material (Bisonyl PVC fabric, technically speaking) which is waterproof and lasts for ages. Though the fabric is waterproof, the design itself is more like 95% waterproof, as water has a tendency to get in through the seams where the fabric is stitched. The bags have a reflective strip stitched onto the front and rear-facing panels and the bag keeps its shape by way of a rigid plastic card material. The top flap is secured over the opening with a pair of sturdy buckles.
Not only are these bags a standard-issue item for any self-respecting Dutch bicycler, you can use them to add a splash of color to a mostly neutral-colored bike.