The cartridge is assembled by pressing in the actuating plunger or pin, placing the shell in the case and then releasing the pin, with the result that the lug of the lever grips the undercut of the shell base. When the cartridge is loaded in the apparatus the actuating pin is automatically pressed in, releasing the lever from the shell, which travels on slightly. On the round being fired, the case is extracted to the rear in the usual way, leaving the shell to be discharged from the front of the apparatus.
Source: Handbook for the Q.F. 3-INCH 20-CWT.
The 'cartridge' is not mentioned in the 1939 6-PR 10-CWT Handbook but the same style of round is pictured and described in the 3-INCH 20-CWT (1940) Handbook.
I'd guess the shell separating from the case allows for more realistic loading/extracting practice.