Why this over the CHIP, or even an actual GameBoy with a flash cart?
Edit: Just noticed someone above linked to https://getchip.com/pages/pocketchip guess it just needs more mAh batteries
https://www.amazon.com/SMART-x30B2-x30FC-x30E0-x30DC-x30A4/d...
You, as an end user, certainly can "just" use a gameboy + a flash cart, or an iOS or Android device, or even (and I would suggest this) code games using an old console emulator as your runtime and don't worry so much about hardware. (Emulators for those same consoles also exist for iOS/Android, after all.)
But this company has other priorities, and so made this.