Brightness, mute, and volume. If you tap the brightness or volume control buttons, they turn into sliders. But you don't need to use 2 separate actions. Simply put your finger on either icon and begin sliding. This is faster and more accurate control than buttons ever gave.
I've also found the Touch Bar to be useful when highlighting PDFs. It you select a bit of text, the Touch Bar shows a few color boxes. Touch one and your text is highlighted.
Also, I replaced Siri with a dictation button, so it's easy to invoke dictation.
And as others have pointed out, it's also useful for video playback control.
Granted it's not constantly useful. But I never used the function keys except for the brightness and volume and mute anyway, and the Touch Bar does those things better. I used to get frustrated whenever an app assigned important functions to one of the function keys anyway. It was lazy UI design and generally showed a lack of thought with key-bindings. What mnemonic is there for a function assigned to F4?