1. So you can keep the shirt and just replace the collar. I have had shirts I loved where the collar points became worn - while the rest of the shirt was serviceable - and I donated them. I suppose if I had "extra" collars, I could have just swapped and kept them. However, the only detachables I see are for standard white/blue shirts, ones that are easily replaced - not like the ones I discarded where the material was unique. Though, now that I think about it, I could have probably salvaged them both with a white "contrast" collar.
2. So you can mix/match collars with different shirts. This sounds odd - I mean, are you really going to pop a blue/white bengal stripe collar on a white shirt?
Are there other reasons that I am missing? Maybe just the old-world novelty of it?