I'd wondered about the U-shaped wedges as well. Been looking for some with no luck even though I didn't know what they were for before now.
To indicate they help you by not needing to jack the vehicle as much since spring will already be compressed is one thing. But imagine the force required to drive them in under the nut to force the spring to compress. And then how potentially difficult it will be to drive them back out. Especially since that area around the nut has a type of shield around the edges to protect it.
Guess that's why we have floor jacks now