Compile into 6502? The CPU was designed to be programmed in assembly, by humans. If your first instinct is to reach for a compiler, you're using the wrong CPU.
I think one great reason to learn 6502 assembly is to write your own Nintendo games and play them on real hardware. Admittedly, that's an extremely niche thing to do, yet there's a thriving community [1] dedicated to the hobby.
The 6502 is still used today in cheap embedded devices, where they are often programmed in an HLL (some C subset is common); its architecture is more similar to that of microcontrollers, which are just as if not more "unusual" and constrained (8051, Microchip PIC) --- and for which compilers exist too.
NES/SNES, Commodore computers, Atari computers (including the 2600), BBC Micro, Acorn computers, and the Apple I-III. There's a reason the 6502 and its variants were the proc of choice for so many computers.
I think one great reason to learn 6502 assembly is to write your own Nintendo games and play them on real hardware. Admittedly, that's an extremely niche thing to do, yet there's a thriving community [1] dedicated to the hobby.
[1] https://forums.nesdev.com/