A capo is the lazy man's guitar tuner.
Dude 1: Ah man, this song's in standard, and i'm in... whatever you call everything a step down.
Dude 2: No need to re-tune, just put a capo on the first fret!
Dude 1: but now the 12th fret nat. harmonic isnt on the 12th fret.
Dude 2: SHUT UP!