A chromatic tuner will allow you to tune any string to any note within the tuner's octave range, so it's a necessitly for alternate & dropped tunings. For example, if you wanted to tune to Drop D down a half step (Db Ab Db Gb Bb Eb), a chromatic tuner would pick up the notes. With a regular auto tuner, you can't do that, because they only tune you to standad tuning. Withot a chromatic, you have to rely more on your ear for alternate tunings.