In UTAU, labels are a simple way to mark sections of the UST to easily jump around. I've been using them to make notes in the UST since my メリュー cover, and for marking sections since my 濫觴生命 cover.
For some reason, I've never seen a UST with section marks, even though I think they're quite helpful.
Label is a note property, this makes it useful for marking specific notes as well.
Using labels
Right click on the top line where the bar numbers are, then select Set Label Here.
To remove or change a label, right click on it again then select the appropriate option.
To jump to a label, right click on the top line, select Move, then choose the label to jump to.
Example
In this UST I've named various sections with labels (Intro, Verse, etc.) so I can easily jump to them. I also have a few labels marking specific notes, for instance here is what I use to mark my progress through the file, and ??? probably marks some notes sounding weird.