It's all about getting a better mixture of fuel and air. With a road car, it spends the bulk of its life at low Rpm which mean low air flows down an inlet runner or manifold. Best place for the injector is at the back of the valve. If it was further out, you can have fuel drop out, basically the fuel ends up running down the manifold wall. Once airflow increases, with the injector at the back of the valve, the fuel spray doesn't have time to mix properly with the air so at that point, it's better to inject the fuel further up in the air stream to get a better mixture. On a race engine you may see dual injectors, one behind the valve, the other outside of the trumpet, the injection will be staged, lower one spraying at lower rpm, the outer one either adding to or taking over from the lower one. All superbikes now have dual injectors as standard. The outer injectors normally have around 12 holes to create a more atomised spray pattern.
I have the secondary shower injectors on my Alfa engine, untried yet, might add a little power, might not butshould clean up the fuelling. More of a tinkering or pub bragging rights thing TBH.
Other thing to note is its not safe running showers and having wasted spark ignition, chance of a big bang or fire. They need to be fitted inside a closed environment, like a yellow fibreglass cold air box, while the injectors spray, you can get fuel standoff which if you've never seen, it's like a big cloud of semi atomised fuel, above the trumpets.
Pic shows hurricane racings modified busso, 400 HP, 10,000rpm, slide throttles and if you look carefully, I think a total of eighteen injectors.
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