Both Aimpoint and ARMS make a half and full spacer for there respective mounts. What are these mounts used for?

The spacers go between the mount and the optic. Differet height spacers are needed depending on if you mount your optic on the stock pictanny rail or an add on, such as a SIR or RAS system, which will raise the optic. If you have a stock weapon it will require a full spacer to line up the optic and front sight. If you add one of the above systems it will raise the optic and require a smaller spacer or none at all. I have a SIR and my aimpoint uses no spacer with that system.