C96 is kinda shitty as a pistol, but with the stock it's easy to use as a pretty good light carbine (for its era), it even has a good forward point of contact with the magazine well, and then stores well inside of its stock.
If there was no SBR laws, holster style stocks would probably be reasonably popular, just because of how compact they store.
I can imagine a lot of scenarios where someone wouldn't have the space to store, or didn't want to carry a larger carbine like an MP5, but then could have something like a Glock 17 with a holster stock, which could be stored in a backpack (like a bugout bag), and then worn as holster for most of the time.
The C96 was originally chambered for 7.63x25mm Mauser (compare to the later 7.62x25mm Tokarev cartridge), and then the Red 9 variant was later a contract run for WW1 when the German Army needed more pistols, and they specified that they wanted them in 9x19mm Luger.
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It's much easier to find a C96 chambered for the original 7.63mm Mauser, than it is to find an actual Red 9 C96 pistol in 9mm Luger. There's a lot of refurbs and mixed parts rebuilds which will be more affordable, and since they're less nice and pristine, you wouldn't have to worry about wearing away on them and ruining their value with regular shooting.