At the range it would be for putting more MOA into the base than I had anticipated would be needed. If I’m going to shoot a rig to 1000yds, I want my 500-600yd zero to require that I’m dialed near or below the mechanical center of the scope. For hunting rigs, I want my 200yd zero to be right at the mechanical center. Why not just buy a base with more MOA? I’ve hunted with match rifles, as well as shot match rifles in different classes that required me to change scopes(to meet weight limits, power limits, was better off not using a fixed 36X, or swapping to a hunting scope). For those, I used Leupold QR rings. Mauser bases with built in MOA are limited. I’m about to use a two-piece ring/base combo and may choose to shim the rear base if I don’t use Leupold, which does offer one with extra MOA built in.
Before the range, I use them to get the base closer to level if I haven’t settled on the optical setup yet and am not ready to bed the base/bases to the action. Remington changed the rear bridge height at some point and all the new Remington mounts are the for the new rear bridge height. Additionally, before they went to CNC, things just weren’t perfect all the time.