Hi Arnauld and welcome!
Push button is connected to GPIO 23 and the led GPIO25, so it is of course optional but if you write a program is could be used to perform some actions (recording for example) without the need of using a keyboard.
There are many controls with the ULTRA and it could be confusing at first. So we have made some ALSA script so you can autoconfigure it to for example preset_external_jack_microphone_input2
to enable the microphone input see here for tutorial