BTM FP are the first to run out, so someone starting the day at the
Disneyland park might take a BTM FP for the evening in late morning, early enough to park hop to the other park and still find some availability. Continue the day at the Studios and switch back to the main park for the evening FP at BTM and then Illuminations.
It's not something I would plan my day on, but if it happens, it's good knowledge that can help improve the day flow. I would not go back and forth between the two parks just to maximize FPs