You've been getting a lot of philosophy lately and not much about the actual craft, so I think it's high time we got into gates and expanders. You may be familiar with a compressor, which waits for the signal level to exceed a certain threshold, and then it automatically reduces the gain by a set amount. In principle they work a lot like compressors, except that they live in an "off" state and hold back some or all of the signal until a threshold is reached then they let it out.
A lot of compressors come with a gate as part of the signal chain, and that can be good or bad. If you really just need to cut out some annoying buzz from an input you can probably get it done, but doing anything more subtle is probably going to require a device with more control than just a threshold knob. A good gate will have not only threshold, but be able to adjust attack and release. Some may have controls right on them that allow you to key the gate to a particular range of frequencies, so your snare drum doesn't open your floor tom gate for example.
So let's talk about applications where you should use a gate and where an expander is more useful. Gates are great for drums. You've got a loud environment with a lot of mics clustered around it and unless you're looking for a specific sound, you want nice clean inputs from each of those mics. A properly adjusted gate on the kick and toms will let you get nice pure signal from each mic, instead of every drum appearing on every input to some degree. Then you can adjust the attack and especially the release to tailor the sound. A lot of drummers like resonant toms, but in a big room with a lot of low end flying around, a tom that says, "DOOOOOOoooooooom" might be more useful to the mixer if it just says, "DOO" and gets out of the way of the next sound coming in.
For instruments like acoustic and electric guitar, you can get handling noise or electrical noise that you want to cut out, but you also don't want your signal to cut in when they start playing like someone flipped a switch. This is where expanders are more useful. A gate has a set amount of reduction and it's a lot, enough usually that it may as well be infinite. An expander allows you to set how much reduction there is with a knob like you find on a compressor. A compressor with a ratio of about 10:1 or more becomes a limiter. A gate with a reduction of 20 dB or less starts to become an expander. It takes some practice to get it all set up, but properly done you can get a guitar channel to open up nicely even for quiet notes and still keep handling noise down. You might not be completely removing it because when the expander closes it's reducing by maybe just a few dB but it's still keeping those unwanted sounds from being up front in the mix.
The first time I really used expanders was the first time I got to mix theatre on a digital console. I was a few days into tech week and had things pretty well nailed down, especially in the loud sections because I had just a ton of dynamic processing at my fingertips. With that nicely dialed in it didn't matter how crazy things got on stage, my levels would stay appropriate without a lot of input from me on the faders. But in the quiet sections I was wishing I could keep open mics on stage a little quieter and keep those tiny noises from adding clutter to some really poignant scenes. Lo and behold there was an expander/gate on every input and I got to work.
I didn't completely shut them down, just had them reduce the level slightly when an actor wasn't talking. They also opened up as quickly as I could get them to, so there wasn't this ramp up when they did start to talk. It took a little while to get them dialed in because it had to sound natural but once I got it I swore I'd never mix theatre on an analog desk again (at least not without a rack full of expanders anyway).
Another time they came in really handy was at a festival stage in a tent. I had a keyboard player who was deaf as a stump and needed a ton of keys and especially his own vocal hot in his mix. With the tent reflecting that back down into the mics it was a volatile situation. But I just turned on an expander and while the band was playing got it to reduce his vocal mic by about 6 dB when he wasn't singing. When he approached the mic he came through loud and clear and when he backed away it pulled back a little, all by its self. That left me free to concentrate on other things and I didn't have to notch out the EQ on his monitor to the point where things started to sound strange.
If you're not using gates and expanders you really should be, it's one of the things that separate the men from the boys in this business. If you don't have the opportunity to practice on real gear, get in the digital environment and make or acquire some multi-track material and practice gating the inputs with plugins. It's one more thing to do when you're line checking a band so you need to be quick, but it's well worth the effort.