The reason to mount it further away is the same as wanting a longer sight radius between front and rear sights.

The further it is from your eye the better your eye will pick up smaller variations in the movement of the front of the gun. Having it close to your eye will allow the muzzle more wiggle room before your dot will pick up on it, and the dot will appear bigger to you and it will be a much more coarse aiming solution.

On the downside, moving the dot too far away will cause more blocking of your peripheral view since it's more of where your two eye pictures converge. Keeping it about midway down the rifle allows both eyes to view more of the target area.

For an example of this, keep both eyes open, focus on a far wall, and cover your dominant eye with your hand. Slowly move it away and notice that you can still see almost 100% of what's in front of you. As you start getting 1ft or more away your hand will begin blocking a chunk of your field of view and your eyes try to focus more on your hand.

So in short, you want the dot as far away from you as possible while still being before your eyes converge. If you're not worried about blocking anything, put it wherever it feels best.