It has many uses really. It's a nice resource for learning assembly, especially 6502 assembly; it's a good reference for creating NES games, whether those are original titles or simply romhacks; it's also a great reference for building an emulator for the platform or a level editor for the game. And of course, disassembling a game and documenting the source code is a great hacking challenge that will leave you confident that you know asm. So, all the reasons that you listed, I suppose.