Most cars on the road have an atmospheric engine.
This means that the motor, without further help from a turbo or compressor, sucks in air thats necessary to burn the fuel.
For most of these engines Water Injection will have little or no advantage in gaining more power.
Except in very warm weather. Then the air contains little oxygen and will additional cooling of the intake air establish an advantage.
But there is a very good reason for using Water Injection when you have a tuned engine or when you have a so-called EGR.
GDi engines or JTS engines have this EGR.
The EGR sends the exhaust fumes back into the engine which reduces NOx emissions.
But of course you understand that thereby the engine inside pollutes and leads to problems.
Water Injection can offer a solution because the steam that is formed by the water cleans the engine.
Look at the video on this page that is specially made for the GDi engines.