i7 7700k with a gtx 1080. it has one fan, different from those that have 3 fans. unsure why its only one fan
my cpu has a liquid cooling with 2 fans
thats my computer
The common practice is not to mention the fans sitting on the radiator - it's understood by default, that the fans are there.
In your case the fans on the radiator are also acting as intake fans, which are complemented by one fan on the exhaust (back of the case). This is realistically all you will ever need.
gpu gets around 80 degrees so the fan in gpu gets loud af
If you are experiencing high temps then an extra fan won't hurt and they are pretty cheap. I am guessing you have a blower style card which draws air from inside the case. The more cool air it gets the cooler and quieter it will run.
i added in 2 intakes and it didint affect anything at all so im returning them