It was a matter of esthetics, gents. That was not at all uncommon, back in the transition era. The riflemen of the day were used to seeing a patchbox (and were handy to carry more than just patches in them) so several of the rifles made during the changeover from percussion to cartridge had the decorative patchboxes included. The "Officer's Model" of the Trapdoor Springfield had the same. As did the H&R reproductions.
Ruffian, you said the KEY part: "I thought those were only needed on the percussion models."
Needed, vs. wanted.
Drifter, I think you'll find that it was NOT added at a later date, rather it may have been REPLACED back to original configuration.
I'm not an expert, but from what I've seen in my research, the Officer's Model of many of these different rifles included the patchbox, as well as SOME of the first cartridge models AFTER the percussion guns.