Because it sounds cool.
The theory behind it, is the further the power strokes are apart, the less wheel spin or at least the better the rider can control wheel spin.
Must be some merit to it, as Yamaha introduced it in their 4 cyl. GP bikes in 2005 and HD still wins races on National dirt ovals with an antique engine.
The new Yamaha Tenere 1200 uses it on their current twin and have been using it on the TDM for years.
There are limits. In the 1920's some vertical twins ran 180 degree cranks. I piston up and the other down. Results in a firing order of 180-540. It also induces side to side vibrations so bad, it would shake the bike apart.
V twins all have it. What the firing order will be depends depends on what angle the cylinders are to one another. Ktm's 950 cylinders are 75 degrees apart and the crank rotation is 285-435 degrees.
If you own a 800xc with 240/240/240 crank, it is all BS.