I2C interfaces are also found on motherboards (e.g. it is used for plugging in a security device like a TPM module) with a pin connector, or for plugging in a LED indicator (that can display boot status, or control "decorative" RGB LEDs, showing the temperature, frequency and some other status, including a realtime clock, sometimes even a small LCD or LED graphic screen showing animations and variable messages).
That connector is managed frequently by a specific bus driver (controlled by a firmware management interface, such as Intel Management Engine, so that it wan even work when the PC is shutdown and no OS is loaded, as it has a specific powering; it may be used to wake up the PC, and this bus interface is also used by "WakeOnLan" Ethernet boards, or keyboard/mouse connectors for waking up on mouse click or some dedicated key on the keyboard, or some specific USB ports but not all; this is a very low power interface ; it may be found as well inside notebooks; this interface generally requires specific drivers for controlling what it will be allowed to do when the OS is not loaded, and it has various hardware security restrictions requiring a specific protocol even if it's implemented on a single small SoC chip).
