Actually both. Some early Colt M1917s did not headspace on the case mouth, only on moon clips, but that was later changed. The main benefit of the Moon clips is to allow faster case extraction and reloading. .45 ACP cartridges can be used without moon clips, but is slower to load the chambers and you may have to pull out each fired case individually by hand - or punch them out with a pencil, stick, or whatever else is handy.