Please explain like I'm 5 why I would use M/s, when it's generally used and where it's used?

Your expertise is appreciated!

 

Here it is pretty well explained

https://www.izotope.com/en/blog/mastering/what-is-midside-processing.html

Here are a few tips and tricks for using Mid/Side audio processing effectively.

Mixing with Mid/Side Processing

• If a track has multiple guitar parts, route them through a bus, using Mid/Side processing on the guitar bus. Automate the Mid/Side tool to boost the volume of the side channel during a chorus, or other section of the track. This makes the guitars sound bigger without adjusting panning, and as a result the section sounds more impactful.
  
  
• Likewise, a slight volume boost to the side channel on drum overheads can enhance the room sound, or a slight boost to the mid channel might enhance the snare drum and rack toms.
  
  
• On any particular instrument recorded in stereo, a high frequency EQ boost on just the side channel makes the ‘wider’ elements sound brighter. A Baxandall filter or a high shelf filter work best. This helps to add clarity to a reverb, without muddying up the signal too much.
  

Mastering with Mid/Side Processing

• If a mix sounds muddy, try reducing low frequencies in the side channel with a low shelf filter. This might be useful, for instance, to surgically EQ the mud out of hard-panned guitars while preserving the vocal and kick drum in the center of the mix.
  
  
• If the mastering compressor is struggling to reduce dynamic range without a perceived narrowing or squashing of the signal, use a Mid/Side compressor to apply less compression to the side channel than the mid channel. Heavy energy in the center of a mix, where the kick, snare, bass sit, can cause a compressor to kick in, which actually squashes the wider, more ambient and spatial elements in the mix. This technique helps avoid that problem.
  
  
• A dry acoustic mix can be warmed up with Mid/Side reverb. Add reverb to the mid channel, but filter out some of the low end on the wet, reverberant signal to avoid muddying the kick drum and bass. On the side channel, add 2-4% more reverb than on the mid channel, with no filtering necessary.
  

 

Once you realize what's the role of m/s, that's one of those things you won't learn on reading but with practice...

 

By no means am I considered a pro mix engineer yet :0 (I managed to get 2 songs on radio so far only in canada and Jamaica) so take this with a grain of salt but I use m/s to clear space in the center of the mix for the low end kick, bass and whatever I feel requires center stage, be it an instrument or vocal in the overall sound and vibe of the song. Then I position the more rhythmical and supporting elements in the side chanels and often raise the volume a bit to create a wider sounding mix. Try using phase shifting (sample delays) on elements that are supportive in the song to push them out to the sides (not too much though as it will cancel out when summed to mono). It will make u r mixes sound clearer and cleaner. My 2 cents!! Best of luck!!