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:
This is the end of the episode, and at the end the host gives an outro, and I play music in the background as she wraps up the episode. You'll see a little yellowish-whitish line follow the music track (the purple track) as the host talks. When she's done speaking, you'll see the arrow pointing to 2 little dots, moving the line up. This means the volume will increase at that point, then I let it stay louder until the music ends.
You can add keyframes by left clicking the line on the file in the track.
You can add keyframes by left clicking the line on the file in the track.