They fit inside the springs and can be inflated/deflated to add extra support and resistance to the suspension. Maintains ride height and prevents the suspension from bottoming out with the weight of a trailer on it, so it can still do its job and soak up rode bumps. The difference in handling and comfort is night and day with and without them, and if you're not towing, just deflate them to around 5 psi or so and it's like they're not even there.
The ones designed for coil springs are universal based on the measurements of your springs. This is the kit that I used for the EUV.
These are the ones I got. There are some different brands that all seem to do the same thing, but this is what I found. Has worked great so far.
Only the one tow, but they've helped a half dozen times level out the back end when loaded down with all our shit and dogs. I think they were totally worth it, and easy to install.