If your fork has mounts for cargo cages it would be a waste to let that go unused. Not the most glamourous part of a bikepacking set-up but these bags add a lot of volume so they might as well look good doing it.
They are constructed with a floating liner, so as waterproof as a sewn bag will get. If you need submersible waterproofness you’re better off looking at welded dry bags, but these bags will keep their contents dry in any circumstance you’d want to submit your wheel bearings to.
The 14cm diameter and 30cm rolled height give them a volume of 4.5 liters.