Good question.
I guess programming GBA Emulators is not as hard as 3DS/DS/PS1 etc. so it's probably for learning purposes. It's relatively easy and you get good results without getting frustrated to much.
There are also a lot of NES and SNES emulators.
Always good to see more options! I think mgba is king in terms of accuracy at the moment, but I'd love to see a GBA emulator with some sort of MSU-1 like functionality. It would be great to be able to use the many soundtrack restorations to bypass the sound limitations of the GBA. Even with clearer output, the samples themselves are crushed to fit the file size limitations.