Hawgz,
The reason is simple.......Options and POWER.
Nobody says that just because a bow is IBO rated at 340 that you HAVE TO shoot it at the top end.
Bows like the BKII, or the Upcoming Patriot DC especially......load em' up with a heavy arrow and whatever string accessories you like and launch away! Do you see any benefit to a 500gr arrow doing about 280fps? I do....especially if a shoulder blade is hit.

That extra speed also gives you the option of drawing a lighter weight bow to get the same performance as one at a much higher draw weight.......my 62# Dual Cam Pro38 was still putting out 73 ft lbs of KE with a peep. loop and leeches. ANd it drew like BUTTER. Any benefit to that on a cold Day?

How about the short draw archer?......Here's the chance to gain VALUABLE energy for performance on game.

The neat thing is that these bows shoot smoothly and quietly even at the top end........you should hear and feel them with heavy arrows. Down right plush.