flipperzero-firmware/wiki/features/GPIO.md

Flipper Zero can be used as a versatile tool for hardware hacking. Its 12 built-in GPIO pins are 5V tolerant and allow you to connect it to any piece of hardware while running your own code, controlling it with buttons and printing debug messages to the LCD display.

You can use it as a handy firmware flashing, debugging, and fuzzing device, as well as USB to UART/SPI/I2C/etc adapter connected to the PC.