What else would an hot red wire be for in a split-tab outlet?
It could be a Multi-Wire Branch Circuit (MWBC).  You can test this with a multimeter -- if there's 240V between the red and the black hots, it's an MWBC.  

These used to be common for kitchen outlets, since often heat making devices need the whole capacity of a circuit themselves, so this technique allows double the circuit capacity with just one more wire.  More recently, it's fallen out of favor, since GFCI and AFCI protection, required by modern code, are more difficult to add to MWBCs.