Spoilers do 2 things: adjust the volume in the calf area and change the effective forward lean of the boot. If you have skinny legs, keeping the spoiler in will help reduce the amount of slop in the cuff area of the boot. If you have big calves, removing the spoiler creates room and relieves pressure.
As you adjust the spoiler, you also directly change the forward lean of the boot, at least how forward your leg sits in the cuff. For example, if you have a "normal" fit in the calf area but want a more upright stance to your boot, you can remove the spoiler. And conversely if you want more forward lean, you keep it in.