Many devices connect through a UART for console access or programming. Many embedded devices' UARTs are broken out through a header with 5V (Arduino) or 3.3V (Beaglebone, Raspberry Pi) signaling.

My LG TV has an RS232 Service Port that can be used to control the TV (including Power On) via the serial protocol. I initially used a Raspberry Pi with a USB to Serial adapter and it worked great.