The Multi-Use Button born from a Brazilian cultural joke better describe like "Turn On the 'I dont care' button".
Below you can read the Apple Store description.

It maked more sense after a "The Big Bang Theory" episode where Sheldon use your pocket alarm.

It allows you to quickly and easily call attention to yourself in case of an emergency or in some other loud environment.
Because of its simple interface, activating the alarm does not require precise movements or even looking at the screen.
This is essential in case you are in a situation where either your movements or your field of vision are restricted.
Also, you can configure the text displayed and the alarm sound of this app, in order to more precisely broadcast your current situation/status as need.