It gives you more flexibility in where to drill your bolts if you aren't using a pre-determined length of chain
Not all manufacturers of hardware makes this kind of set up so if people aren't buying hardware from you or already have hardware from other manufacturers they can do this.
Also, I didn't realize you can put the pig tail straight on a regular hanger, I thought that only worked for glue-ins.
They fit any chain you can get a 10mm bar through and every known type of hanger (except pop-tops where the orientation would be wrong).
The only time there are problems are with glue-in bolts or staples which have been installed too near the rock (like Fixe bolts which have been recessed too deep and no longer conform to the standard) but a) that´s not my problem b) they will fit if your hammer is big enough.