Different chord voicings become playable where they would be near impossible to play in standard or other tunings.
The list below gives tunings from standard guitar tuning as well as many others to experiment with. The first note of each tuning listed below represents the 6th or lowest string (low to high).
E - A - D - G - B - E (standard)
Eb - Ab - Db - Gb - Bb - Eb (standard a half step down)
How to Tune a Guitar to Standard Tuning
These tunings allow the guitarist to create a power chord by barring one finger on the 3 low strings at the same fret. The lower a dropped tuning is the guitar strings will get looser and can rattle on pickups or frets. Some guitars will need to be intonated to prevent this.
Dropped D = D A D G B E
Dropped C = C G C F A D
Dropped G = G D G C E A
Dropped A = A E A D F# B
Open tunings tune the guitar to the same notes of a chord so when all the strings are played open they play a major or minor chord. Open tunings are popular for slide guitar because you can hold the slide on all the strings and modify finger position easily.
Open C = C G C G C E
Open D = D A D F# A D
Open D minor = D A D F A D
Open G = D G D G B D
Open G minor = D G D G A# D
Open A = E A C# E A E
Modal Tunings are similar to open tunings but don't tune to a minor or major chord - modal tunings typically tune to a suspended or other chord form.
Modal D or Dsus4 = D A D G A D
Modal G or Gsus4 = D G D G C D
D5 power chord = D A D D A D
E5 power chord = E B E E B E