About alternate guitar tunings...

The subject of alternate guitar tunings is a fun, and often controversial one.
To be clear, remember that the term "standard" is all relative.
Generally the Six-string E-A-D-G-B-E tuning is considered "standard" in most guitar contexts, 
however there are literally hundreds of ways to tune the instrument and make good music. 
There are also guitars that have more or less strings in addition to a "non-standard" tuning format.
When approached on the subject of playing an alternately tuned or say 7-string instrument, 
the average guitarist will often remark: I can barely play my 6-string E-A-D-G-B-E, 
why would I want to increase my headaches?
This is one way to look at it, but always keep in mind that there are many ways to 
view any subject and an open mind can be a valuable thing in life.
Although obviously not for everyone, there are some credible reasons why you might want 
to consider experimenting with alternate tunings or non-standard instruments.
1. To expand or change the available note range on the instrument.
It is possible that as a musician, you need (or want) to make music using notes 
that are outside of the range of a "standard" strung and tuned guitar. 
This is one reason why 7-string B-E-A-D-G-B-E tuned guitars are gaining popularity.
The addition of the low "B" string allows players to create riffs, melodies, and even chords
that would not otherwise be possible. A number of modern "Rock" and "Heavy Metal" style guitarists 
have adopted the 7-string guitar and this tuning combination as their primary "standard" instrument. 

Jimmy Bruno an awesome jazz guitarist, plays much trio work and uses a 7-string tuning: A-E-A-D-G-B-E. 

Interestingly, he uses the low "A" primarily for "jazz" bass type riffs and lines although the tuning allows 
for some really unusual and nontraditional low range chords / harmonies.
Originally, when I read about his tuning I wondered about the logic of Jimmy changing the low "B" to an "A". 
Once I charted the tuning using M.A.M.I. it was easy to see the logic and purpose of such a change 
especially from a harmonic standpoint. There are a great many practical uses for this tuning I discovered.
As accomplished as Jimmy is (and he is tops!) I presented him a copy of the M.A.M.I. Scale Atlas with the 
hope that he might see some more uses (particularly harmonically) for his a for his unique tuning.
2. To allow for easier, or impossible fingerings on the instrument.
If you study the guitar long enough, you will realize that some chords and lines are very difficult or not 
possible to play using the standard E-A-D-G-B-E tuning. It is possible that you might need (or prefer)
to arrange the notes differently on the fingerboard using a non-standard tuning to accomplish the "impossible".
There are a number of interesting tunings to this effect. "Open" tunings are often used to simplify chordal and harmonic 
playing throughout the range of the instrument by reducing the amount of hand / finger movement necessary 
to create the desired chords. Strange as it may seem for most guitarists, sometimes an "alternate" 
or different tuning can help you play better...or play the impossible!

An open mind is important in life...if you combine this with a bit of extra study on the instrument, anything is possible.