Why would anyone want to?  Well, for fun, maybe.  Or if a violin/viola/cello/mandolin/etc player has a guitar around that they don't actually use, it would allow for practising chords and etc on a fretted instrument to work on your ear a bit.  And it sounds kinda neat. 

Normal guitar tuning is E A D G B E.  Mandocello is tuned C G D A.  It's is a compromise, but I came up with a tuning that would basically be sort of a 5 string mandolin tuning, but down an octave. 

The tuning I'm playing with on a guitar is C G D AA E.  From what I gather, mandocellos usually have 8 strings, tuned in pairs.  This trick tuning would only give a doubled string on the A, but it doesn't need buying special strings or modifying the guitar.

Tune the guitar's low E down to C.  Tune the A string down to G, leave the D tuned as it is.  Then tune the guitar's G string up to an A and tune the B sting down to match that A note.  Leave the high E tuned as it is in normal guitar tuning.

That's it. You can get some neat sounds with it, and can use violin/viola/mandolin/etc scale and chord patterns. 

I got the idea from member Raywells mentioning mandocellos in chat, and remembering that acoustic guitars are tuned almost as low as cello.  So I looked up mandocello tuning and puzzled it out.