Most "bigger" SoCs have a secondary processor for SoC bringup/boot. (It's not uncommon to see a tiny ARM7 or something in this role). Since Broadcom succeeded in making a particularly general purpose core for their GPU (it seems that the GPU runs a full RTOS written in C), it only makes sense (well, to me at least) that it could take over that role as well to save on the transistor budget.