Exam mode, or test mode. It's something that appeared about ten years ago, to ensure that a graphical calculator isn't loaded with cheats or has certain features enabled. The technical reason is that the RESET button no longer clears all of the calculator's memory (think Flash, not RAM) and proctors like to see a flashing LED that tells them everything's fine.

It's a flawed idea and has led to an arms race, where manufacturers lock down their models and jailbreaks break them open. Even NumWorks, who originally had a calculator that was completely unprotected and used to publish all of their source code on GitHub, had to give in and introduce a proprietary kernel and code signing, in order to stop custom firmwares and applications from accessing the LED and stop countries from outlawing their calculators.