Outside of the few flashcarts I have, don’t really know the history behind the creation of them. Probably worth looking into the everdrive made by Krikzz as a starting point.
My Life in Gaming also did a few videos on the subject a while back. Here’s one of them: https://www.youtube.com/watch?v=fuHA3k-y6PE
For me personally, I think it’s a good way to play rom hacks on original hardware. Definitely a good way to also not put as much wear and tear on the cartridge slot either