The advantage to a left-side function key is that it makes it faster and easier to access the media keys 1-handed, in particular when your right hand is on the mouse and you're trying to keep your left hand as close to WASD as possible. Useful for a "gaming keyboard."
Bzzt.
I've actually talked to the folks who designed the layout. That's not why the Fn key is at LWin position. Nope. Not even remotely. (Both SteelSeries and Thermaltake use this layout, FYI. And yes, I talked to folks in the know at both.)
The reason the Fn key is located at LWin is because that's below WASD, and the most frequent 'accidental Windows key hit' location. Normally when gaming, you have your left hand on the keyboard and right hand on the mouse. That means you have pretty much no chance of accidentally hitting RWin/RGUI, but if you slip on Shift or Ctrl (both frequently used keys) you're likely to hit LWin. The media key layout was more along the lines of happy accident (default locations on Thermaltake/CST-104 Sydney.)