You are confusing the cartridge head, behind the extractor groove, for a rim. A rimmed cartridge has a flange that protrudes beyond the body of the cartridge (rim diameter>case diameter).
The use of a moon clip allows for positive headspacing and extraction. There are those that will tell you that you don't need clips. It has been my experience that in that instance, ignition is far from a sure thing. Depending on chamber dimensions, chamber finish and cleanliness some rounds may be held in place long enough for a hammer blow to ignite the primer. Others will be driven forward in the chamber the result being a hammer strike of insufficient force to create ignition - a light strike.
While it is true that there is a "step" machined into each chamber in the cylinder and theoretically the cartridge could headspace on the mouth of the cartridge against this step, factory brass and ammo are generally made to dimensions shorter than SAAMI maximum - which is the dimension used for the step to insure usability with all SAAMI-spec ammo. Semi-autos are able to function because in most instances they actually headspace on the extractor, not the step in the chamber.
Rimless (most semi-auto rounds) and semi-rimmed cartridges (i.e..38 Super) used in a D/A revolver, require moon clips for positive ignition and extraction.
Adios,
Pizza Bob
Edited to Add: There are exceptions to the necessity of moon clips. As stated before there have been D/A revolvers designed from the ground up to preclude the use of moon clips - the S&W 547 most notable among them. Charter Arms current series of Pit Bull revolvers also have special extractors so that clips are unecessary.
