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the main reason is to move the drive side spider away from the frame (in my case).
As howard illustrate very well here, you can move the drive side by ading spacer on the non drive side. That way i moved the drive side away from the frame
When you add a spacer under the non-drive side cup, you actually move the spider closer to the chainstay.
If you really want to move the drive side crank further outboard and still have a GXP crankset properly assembled, you have to face material off the NDS of the BB shell. The whole left/right alignment is determined from the NDS face of the shell, as the NDS bearing is located with reference to that, and the axle is located with reference to the NDS bearing.
gbj_tester
10k miles in total on my both bike with spacer (1 & 2 mm) and it works perfectly. A good chainline allow you to reduce noise and smoothness but the main reason is to move the drive side spider away from the frame (in my case).
As howard illustrate very well here, you can move the drive side by ading spacer on the non drive side. That way i moved the drive side away from the frame otherwise it was rubbing very slightly the frame. A 2mm spacer and you get the necessary clearance.
Spider on omnium are rather bulky and many people bump into that problem, adding spacer allow you to run it and the tolerance of the crankset allow it perfectly. As howard say above, the bolt which lock dive and non drive side is only there to be tighten until the crankset lose any play.