Lots of Triumphs have been done like this, mostly in drag racing. I suppose someone doing it on an XS would be making a "nod" in this direction.
Original benefits or reasons might have included the ability to fit a range of carburettor options out in front. For a twin-engined drag bike flipping one of the heads might make it possible to fit in two blowers (one for each head). I've heard it said that the front facing bell mouths result in increased induction. Frankly this sounds like someone guessing what the reason might be, as I doubt it would work. (My guess is that an engine would prefer still, unpressurised air or over pressurised, but turbulent, air.)
There is an easier, straight run for the exhausts on a drag bike. On anything that runs for more than 9 seconds at a time, though, things are going to get very hot.
If you want to run a single carb on an XS (and value symmetry) then it could be a valid reason. (frame's in the way out back)
But hey, who needs a reason? Why would anyone fit springer forks? (other than they think it looks cool or they wanted a challenge).
Dave