With attack and release, you can shape the sound. That's something you can't with volume sliders.

Synthpads are compressed heavily to keep them audible although they are in the background. Because you can 'even out' the sound of strings and pads.

I guess there are more examples but the things mentioned above are my experiences.

A compressor is a kind of volume automator but it works in milliseconds, something that is virtually impossible to do with automating sliders.

And yes, you can compressor several layers together to get a glued sound.

Don't negate the possibility they may not know what they are doing.

It depends on the synth dynamic and the intended application.

You can control the volume of the sound over time as the timbre changes. For example, a sound that starts out as a high frequency and then goes down to a low frequency may have a different amplitude throughout this change. Compression will bring these high volume parts down to the low volume parts, ultimately allowing you to have the sound at a steady volume throughout the mix. This maintains the focus of the track as your attention isn't suddenly drawn to a quiet section (or vice versa) in a bassline or synth part.

When you understand what a compressor does, you can apply this to any situation. Whether it be a drum, bass guitar, a supersaw or your uncle's foley collection. You'll be able to assess the situation and apply your own discretion.

By adding a compressor you can get that characteristic sharp attack that you hear in lots of released tracks.  Some compressors also add color.  And last but not least, you can bring up the release/reverb tails to get that typical pumping sound.  I'm not sure if everybody knows the details of how a compressor works, but you can just throw it in and see if it sounds better - most of the time it will (depending on the settings and on your style of music of course).

I use it on my drop layers usually to bring out the tail of the sound or to bring out the reverb.

A lot producers use multiband compression to push the bass frequencies up and make the sound fatter as well as using the compressor to eq the sound like give more presences to the highs cause a lot of synths dont have good highs. I think a lot of people say like lay off the compression but you can pretty hard with the compression and eqing on synths not so much drums and organic sounds but synths. As for the compressor its usually just to glue sounds athough eqing/sound design probably more important.

Don't negate the possibility they may not know what they are doing.

It depends on the synth dynamic and the intended application.

You can control the volume of the sound over time as the timbre changes. For example, a sound that starts out as a high frequency and then goes down to a low frequency may have a different amplitude throughout this change. Compression will bring these high volume parts down to the low volume parts, ultimately allowing you to have the sound at a steady volume throughout the mix. This maintains the focus of the track as your attention isn't suddenly drawn to a quiet section (or vice versa) in a bassline or synth part.

When you understand what a compressor does, you can apply this to any situation. Whether it be a drum, bass guitar, a supersaw or your uncle's foley collection. You'll be able to assess the situation and apply your own discretion.

Me - "I compress my Boss everyday" lmao … Na, but really - 90% of these cats sharing tips don't know what they're talking about - Their mix goes to OTHER engineers who shape up the sound the label wants lol … The industry is funny. But yeah - I mainly use compression on drums, and every once in a while if I have a synth that changes dramatically but aside from that I stay away from compression. I am however about to compress these dishes into my dishwasher lmao ..