Keyframes are very useful in the Multitrack for editing a podcast with a ton of clips consisting of music, intros, outros, ads, etc.
With keyframes, you can manage the volume of a selected track. I personally use it a lot for music, as sometimes I want it to be loud when someone isn't talking or quiet when someone is talking, and to fade up or down between clips.
Here's an example of how I'm using it for a podcast episode:
You can add keyframes by left clicking the line on the file in the track.