I've seen people do it purely to show off, like playing with the guitar behind your head, turning the guitar around and playing left-handed or flicking your pick bouncing it off the guitar and catching it again (still don't know how that one works, my pick just flies off randomly). In that case it's just showing off that you can play the melody in an usual position and also backwards with respect to your normal fingering.

In your example though I think he plays with the left hand inverted for a reason; He is holding a chord on the low strings at the same time needing the high strings ring. If he held his hand in the normal position there is a chance that one of his fingers might touch the high strings it's reaching over and therefore mute the note. He may have decided to fret the chord like that so that there is no danger of interfering with the other strings.

I hope this made sense... (All of the above is pure speculation on my part)