Chorus thickens a sound by kind of mirroring the original sound at a lightly different frequency. You can create natural chorus effects for rhythm guitar by playing a powerchord riff and bassline (tuned to 4400) then reproducing the same riff at say 4350 and 4450. The fact that the lower and higher takes are 'out of tune' to the same degree means they almost cancel each other out and have fattening effect on the sound. I'm almost sure that Metallica did that on the new album (not that I'm fond of the sound), it sounds out of tune yet in tune...