Taking a step back, the concern was with a hangfire, more common in the old days than today. If the cartridge does not go off, and the bolt is opened, a hangfire will blow the cartridge to pieces, wreck the rifle, and drive the bolt back into the shooter's hand or face. By providing a means by which a "dud" cartridge can be retried without opening the bolt, the danger of a hangfire can be minimized.
It is not commonly known, but the M1 and M14 rifles have the same capability. By unlatching the rear of the trigger guard and using it like the lever of a lever-action rifle, the hammer can be re-cocked and a dud primer re-tried without opening the bolt.
Even the M16/M4 can be re-cocked without opening the bolt by dropping the lower and cocking the hammer by hand, then closing up and trying the round again.
Before anyone asks, it is unlikely that the capabililty was or would be used in combat, but it was a serious concern in the early days.
Jim