The side button on iPhones has several well known purposes.
One click turns the phone screen on or off. Holding it is involved in several actions, including turning the phone off with the volume up button and the side buttons simultaneously.
Two clicks activated Apple Pay and gives you access to cards in your digital wallet.
One lesser known feature is that the Accessibility Shortcut is available on iOS by means of triple clicking the side button.
There are multiple features you can map to it.
To access this menu, navigate to Accessibility and then Accessibility Shortcut.
Reduce White Point is available under Settings > Accessibility > Display & Text Size.
The end result is a much dimmer screen that is useful when in bed, at a movie theatre, or anywhere you’d like to have the screen be darker than the normal brightness slider allows.
I am sure there are other useful uses of this shortcut.