The rear shock mount takes every hit the suspension does not absorb. On a truck that gets landed, that is a lot of load arriving repeatedly at one bracket on the frame.
It shows up as cracking around the mount, or the mount itself deforming. Either way the shock is no longer held where it should be, which changes how it works and puts the load somewhere else.
These mounts sit in the same place as the factory ones but carry far more material. Same geometry, same shock, considerably more margin — and fitting them is cheaper than finding out what happens when a mount lets go at speed.
Worth pairing with a rear bump stop kit, which reduces how hard the mount gets hit in the first place, and a limit strap kit, which stops the shock taking the load at full droop.