Meant to quote this one in the above post sorry...
For field shooting and hunting shots, lots of folks want their hand "under" or "squeezing" the rear bag instead of their off hand controlling the gun through the shot process.
This is something I started teaching over 15 years ago for guys who wanted to learn general "hunting shooting" fundamentals which was followed up with live target fire (varmint hunting).
In varmint hunting, you are constantly shooting, racking, moving to a new target, and listening to your spotter for call outs. This is 100's of engagements on small moving targets in rolling hills/canyons in a single morning. When you take guys who have learned to properly control the rear of the gun with their off hand, versus guys who "bag ride" the difference in kill count and shooting ability is stark.
This doesn't at all mean that guys can't make good hits on animals at range using other rear bag methods, it's more about fundamentals, consistency shot to shot, controlling the gun, etc.
Benefits in no particular order...
.More control of the gun
.Faster and more controlled racking of next round
.Improves shooter ability to stay on target during recoil for feedback on target or ground near target
.Improves the common desires for "too much" or "too little" face pressure on stock
.Improves spotting of shots
.Vast improvements in moving body and gun to next target
.Micro adjustments with hand controlling gun versus hand under or squeezing bag are more reliable/repeatable.