Two reasons.
See if you knock at 15 PSI.. and you activate the system at 15 PSI.. you will knock until the alcohol hits the combustion chamber.. think of it as ping ping ping... then clean. So by activating it earlier, by the time your at 15 PSI.. there is alcohol in the chamber. Understand if your motor knocks at 15 PSI it doesnt start to knock instantly.. it needs to be at 15 PSI for a brief amount of time before it does knock. So starting the system earlier cures the knocking at 15 PSI.
Now here goes the next issue. Lets say you need 1 oz per second at 15 PSI to control knock. You activate the system at 8 PSI to assure you have spray when the boost reaches 15 PSI. Now at 8 PSI 1 oz per second is too much and cuases the air fuel to drop hard. So the motor bogs when it activates at 8 PSI. This is why progressive controllers are needed so they can spray .2 ounce at 8 PSI and 1 oz at 15 PSI. And when the boost climbs to 20 PSI.. now its spraying 1.5 ounce... and when it creeps to 25 PSI.. its at 2 ounces per second. These numbers are thrown out and to be used for illustration purposes.
One last one.. if your boost is lets say 15 PSI and your pump pressure is 50 PSI thats a 35 PSI differencial. When your boost goes to 20.. its down to 30=less. Or at 8 PSI 42 PSI=more.. so at lower boost levels you actually overwhelm the engine easier due to the added pressure. So the system when on fixed pressure actually works backwards and gives less as the boost rises.
Hope this helps.
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