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Such 'snail' drums were devised for Luger pistols as a way to turn them into more capable weapons for close quarters fighting. As they had to work in existing pistols, the design is a drum where the single stack coils up. To load these magazines, one needed a tool to be able to wind the spring and to insert cartridges, it was pretty much impossible to do by hand.
These magazines were chosen for the MP18 as Germany were in such dire straits for resources at this point that they absolutely did not want to set up new production of new magazines when these drums existed, thus the resulting gun. When using the drums with the MP18, you would have to slide a spacer over the 'tower' of the magazine, otherwise they could be over-inserted.
If you wonder how the cartridges transition from the drum up into the magazine well through the sharp angle you see there, so do I. As a result of the terms of the Treaty Of Versailles, the majority of these drums were actually destroyed, so they are rare today, an example in good condition can go for thousands of dollars.
