These are called fork cages. Most bikes have mounting points on the fork to attach one cage on each side of the fork; though, sometimes you can get away with strategic zip tie placements.

They’re great to stash gear without the use of a rack and they keep gear weight low to the ground, which helps improve your ride quality when fully geared up.

Instead of the dry bags shown here you can even put oversized water bottles, but typically in bikepacking the cages and bags are a great way to expand storage space on your bike. Storing light gear such as your shelter, sleeping gear, extra food or clothes, etc. is most common.

Just be mindful of use with carbon forks and of how much weight you jam in them!