I think it's intended for checking the same bit in multiple bytes. You load the mask instead of the data.

So much 6502 ASM involves turning your brain inside-out... despite being simple, clever, and friendly. Like how you can't do a strided array sensibly because there's no address register(s). There is no "next byte." Naively, you want separate varied data at the same index is separate arrays. Buuut because each read address is absolute, you can do *(&array+1)[n], for free.

What I really miss on NES versus Game Boy is SWAP.