The idea was that a second stage that was full of silt, had a hole in the diaphragm, etc could be quickly swapped with one off another reg underwater. It's one of those ideas that sounded good but wasn't.
Then there was the crowd who advocated removeable covers so that silt, rocks, etc could be cleared underwater. That idea is just as dumb but still has ardent supporters. How well it works in the real world varies with the design of the purge cover, and even at best, assuming parts don't float away in open water or get lost in no viz conditons in a cave or wreck, diaphragms that are assembled wet have a bad habit of unseating, and the result will be an unbreatheable reg requiring you to do it all over again.
Prevention makes a lot more sense and if you are going to dive in conditions that severe, then bagging the reg in a nylon stocking prevents any issues with gravel in the reg anyway.
The modern counterpart to swapping second stages is the idea of swapping an entire first and second stage in water. If you really need to do that, you've already either had multiple failures or badly screwed up the gas planning and contingency planning.
I've found that most thinking divers at some point in their careers stop listening to "experts" and start thinking through advice, techniques, etc to see what really makes sense versus what does not.
