You’ve used gates, right?
If you’ve ever recorded something with a microphone, you’ve probably had a situation where the background noise was just a little too high (or the acoustic sound was just a little too quiet) and the background noise was bugging you in the mix.
So you reached for a gate, eh?
Set the threshold, maybe adjust the attack and release (I usually prefer an instant attack and fairly gentle release) and call it a day. Bingo! No noise in between playing!
That usually works fine for busy mixes where there’s always something going on, or there is a rich background texture. But in sparser mixes, you might notice that the difference between the silent and non-silent sections of a track is rather uncomfortable. When the instrument is playing, you get a rich subtle ambience (or refrigerator hum, if that’s how you roll). When the instrument stops, the life gets sucked out of the track.
Pure digital silence.
What an expander does is reduce the level of the background noise without killing it entirely. It makes the quiet stuff quieter. It helps you push the background noise out of the way when it’s not needed, but doesn’t totally suck the life out of your track.
Just be careful that you’re not using heavy compression after the expander. Expanders can be quite subtle in their effect, and compressing the resulting sound can destroy your careful balance of threshold and ratio. If you like following your tail in circles, lower the threshold of the expander to compensate for the heavy compression. In general, though, it’s not a good idea to have two processes fighting each other. If you want to use an expander, it’s probably because subtlety is important to you. Use a gentle compressor.