M43 T offers a way to manually control pins with gcode.
Sending M43 T is a variant of M43 that allows you to cycle through your mainboard pins, pulsing them for a specific amount of time. I have not tested this personally, but the example linked on the Marlin reference page should illustrate it's use.
-M42 can be used to manually control mainboard pins. With the correct parameters, you can override the firmware and manually activate connected hardware. With the right gcode (which I will not include here), I managed to turn on the MOSFET for the hot end which rapidly started to raise the temperature without any target, hence the warning further up this page.
Perhaps a better use for M42 is to control hardware that is not supported by Marlin. Where some components are directly supported and have corresponding gcodes, M42 would allow you to still use gcode to trigger pins, even using PWM control.