Mid-Side EQ is, for me, like Cajun spice. You can put it on anything to make it better. Some might say Cajun spice is too salty. It’s not too salty. These people just don’t know when to stop pouring.
Anyway, what Mid-Side EQ does is take your track, and splits it into a “middle sound” and a “side sound.” The side sound is the mix with much of the bass removed, and it is pushed to the edges of the stereo space by setting Width in a Utility device to 200%. The middle sound is the rest of the mix compressed to mono by setting Width to 0%. This means that all the stereo effects in your mix, such as manual panning, phaser, auto pan, ping pong delay, etc., will be exaggerated by contrast, because the majority of the sound is monophonic.
All that really matters is that it makes everything sound crisp, clear, and crunchy. While I wouldn’t recommend it for every type of sound, it fits in most places.
Here’s an example using one of my tracks. Here’s the “mid” channel:
And here’s the “side” channel:
And here they are together:
Compare both of them together with the sound of just the middle channel to see the difference in sound you get. For more information on how to use the rack, check out my tutorial on default templates, which mentions the EQ rack.