This person suspected that this metallic object that came with his bike was a drink holder, as anyone would. The only problem was that there was no place to place your drink. After asking on Reddit what was up, he received an in-depth explanation of what derailleur guards were.
Derailleur guards are something like PPE for your bike. They protect the gear and chain when you inevitably toss your bike to the side when chasing bad guys. They’re not 100% foolproof, but having one is a lot better than not. Derailleur guards typically come with more high-end bikes, so the OP must be rolling in dough.