To the professional Engineer/Producer chorus is a very important effect.
Chorus, like delay, phase shifting, and flanging, were effects that were invented
on the tape based reel to reel recorder.
Chorus and flange were originally discovered by the engineer lightly touching the left and right tape reels.
This would cause the tape to slow down and speed up modulating the pitch of the instrument.
The result was a watery modulated sound that when mixed back with the original instrument sounded amazingly very good.
This effect was developed over the years and now we use digital devices instead of tape machines.
There is actually many useful applications for chorus and flange.
These two effects are very similar, the general difference is flange has more extreme pitch modulation.
Chorus is an excellent tool to know how to use. Many engineers are familiar with chorus as a tool to give a clean guitar a more warm and lush tone but there are many more uses to this wonderful effect.
Chorus can literally be used on any instrument or vocal with interesting results.
One common use of chorus by professional producers and recording engineers is on back ground vocals.
Say you have a song with four part harmony vocals. You notice that the pitches of the vocal parts are just a little off.
You can run these threw a chorus effect just a small amount and it will give the perception that the vocals are all in pitch together.
Chorus can also be used on a lead vocal. When used on a lead vocal try just adding a very small amount of the effect so it is hardly noticed.
Even with a small amount of chorus the lead vocal can seem to have more life and again better pitch.
Chorus is very often used on electric and acoustic guitars.
There is nothing like the sound of a silky acoustic guitar. Again just a small amount of the effect can go a long way.
Because chorus should be used as a send effect you will naturally get a slight doubling between the original signal and the modulated one.
This result helps the chorus to thicken the original track and warm it up.
It is this doubling and modulating combination that many producers use to create a recording that is more warm and rich sounding.