... and the Metronome as well?

The metronome fader is for, well, the metronome. Its where the audio for the metronome is routed and gives you a fader to grab and quickly adjust its volume.
 
I don't know what other people use the preview fader for, but I use it when I'm importing audio. With the import audio dialog open, you can play the wav or mp3 files before you import them to verify you're importing the file you want. By routing that to the preview fader I'm able to turn down preview volume which I find is usually super loud otherwise.

I use the preview fader for auditioning files in the Browser.

Cool! Thanks guys!
I have a use for them now! 😁

Since I use the Preview bus like Chuck and Johan, I would highly recommend lowering that fader a few dB and saving that as a project template.   Many loops and one shots are loud.

Remember to NOT to route the metronome bus to the Master bus.  If you do this, that Audio from the metronome will be recorded in your project when recording.   
 
I usually go into Preferences > Metronome and set the output to main outs.

Thanks again!