Essentially, the gameboy expected a bitmap of the Nintendo logo to be present on the cartridge rom, and was shown on screen at boot. It had to match a version stored on the gameboy itself or else the game wouldn’t start.
The thinking (that I’m not sure was ever tested) was that someone producing a game that tried to trick consumers into thinking it was an official Nintendo product, would be liable for damages in a trademark lawsuit. Since the game would never start without an official Nintendo logo, the hope was to make the legal system enforce Nintendo’s licensing scheme.
