> Is there any reason, at least immediately, to inject code into NES Tetris?

It’s somewhat obscured by all the technical details but doing this exploit does have a practical purpose: it allows highly skilled players to play longer since they can now have a way to prevent a crash that prevents them from playing past certain points. For the average player there’s no practicality to this but for those who want to compete for the highest scores this solves a limitation and opens new opportunities for competition.