Because it's designed for 30A, your fan probably only uses 300mA (1/100th of the current) so it's really overkill but will work. Most motor controllers will be designed for large motors, there isn't much demand for small motor controllers.
Only if you do something stupid like connect a 12V fan to a 24V supply.
With a 24V supply the controller can power a 720W fan, nearly 1HP and with a 12V supply it can power a 360W fan. A 720W fan will probably blow your computer to pieces.