Afaik bitcode is so that Apple can rebuild binaries for different target architectures (e.g. new models of phone, watch, et c) without source developer interaction. It should come in major handy in the OSX app store when ARM64 Macbooks ship in a year or two. A full app store of working apps on hardware launch day will make the bitcode requirement worth it when it's the smoothest architecture transition they've ever done (out of 680x0->ppc and ppc->x86).