The air is not distributed equally to the cylenders due to the path it has to follow - If you give them your full engine spec, they will tell you where exactly to put them. This balances out the engine - reducing internal imbalances increases HP, and the actual increase in performance increases the Hp, so you win twice! And for racing this is perfectly legal! Cleaning the injectors that were factory fitted, and replacing them un altered in the specified holes gives a measureable performance increase, unless you lucked on them being perfect from the factory!
A cleaning does wonders to the performance anyway!