That boom was originally designed to 'reef' the main by rolling it down onto the boom... Theoretically you could 'furl/store' the main that way too. Turn the worm gear and roll the sail around the boom as it drops. Not great, though esp since battens are generally perpendicular to the leech, not parallel to the boom.
Cumbersome, not good for final reefed shape, complicates the vang in reefed conditions, and generally not done anymore expect for 'in-boom' furling which is much better engineered than these older setups.
Most such booms have long been converted to slab/jiffy reefing and the rotary mechanism not used.