If you mount a fan to a spot in your computer case that's meant for another fan, and that other fan is already mounted in the case - either oriented to blow air out or to suck air in - it can cause the air flow of both fans to work against each other instead of in sync. This can create a build up of hot air inside the case, reducing its cooling efficiency.
A system case fan, also known as a CPU or power supply fan (PSU), is an important component of your computer for providing airflow within the case to keep the components cool. The intake fans, which are located at the front of the computer case, pull in cool air to reduce temperature and exhaust the hot air that's generated by the processor, GPU, hard drives and other components in your PC.
Exhaust fans, which are positioned at the back of the case, help to eliminate the heat that's trapped in your PC case by drawing out and expelling air out through a vent at the bottom or rear of the computer. To ensure the proper functioning of your system, it's necessary to orient your intake and exhaust fans to create a smooth flow of air through your computer case. You can determine if a fan is oriented in the right direction by looking at its blades, which are often printed with an arrow that indicates the intended airflow direction.