I don't know who came up with the idea of using standard bottle cage mounts to attach a cage capable of storing all sorts of items, but it is brilliant and another invention that makes bikepacking great.
What is a anything/stuff cage: generally speaking these cages are similar to water bottle cages with the difference that they are made to store larger items. This can be a 1L - 5L water bottle or by using a stuff sack/dry bag in conjunction with the cage you are able to store a wide range of items.
How and where to use the cage:
Most people attach the cages to the front forks or underneath the bottom tube. These two methods of attachment use clamps to attach the cages to the forks or use screws when mounting spots are available, normally on bikepacking specific bikes.
Use tape or an old inner tube to protect the forks when using clamps.
I recently purchased a Specialized Burra Burra Stuffcage to use as storage for my canteen on bikepacking trips.
The cage is made of aluminium and has a low profile. As you can see a 2L bottle will fit to the down tube without it interfering with your tire. I have not tested the 2L bottle and suggest to do so before embarking on your journey.
Why use a cage as storage: weekend bikepacking trips need no more than a bikepacking bag set, I will even go so far as to say that you can travel for a week, but for longer adventures additional storage might be needed especially if you are planning to wild camp and prepare your own food that you need to carry on your bike.
Also perfect to store water in areas where water is in short supply, I have seen folks carry 5L water containers on fork mounted cages.
Benefits and drawbacks:
The benefits are more storage for even longer journeys.
From my experience there are two drawbacks, things that can be improved on.
One: the fastening options are limited to screw mounts. Meaning that if your bike frame does not have holes to mount the cages to, you will have to DIY your cage to your bike with straps/clamps. It seems that the manufacturers of bikepacking/touring bikes developed the cages and other accessories purely for use with these type of bikes.
The truth is however that most use their current mountain bike and can do without the frustration of attaching a cage DIY style after the purchase.
Two: Fastening straps are strong but sometimes lack grip that can cause movement of bottles. The Burra Burra Cage enclosed their cage straps in rubber to prevent movement but after some use the rubber layer stretches and fastening the straps becomes almost impossible.
I think of it as a car manufacturer developing a vehicle but neglecting driveability!?
The conclusion is that storage cages are a great addition to any bikepacking rig as long as you know it is not perfect. Not being a main stream item let's just say it should get better over time. What do you think?