FanGUI gives you control over your fans by allowing to specify at which temperatures the fans will kick in to cool down your system.
It already comes with a default set of numbers for no/low/high fan settings, so you don't have to do a thing and your notebook will run just fine.
Just that sometimes, what FanGUI sets makes more sense than what the default laptop settings are. For example, when I had my i8600, the notebook would get to around 50C with no fans, then the fans would go on high and I'd hear this annoying "whooosh" sound. With FanGUI, I was able to set it where at 45C, the fans would come on at low and cool it down to ~35C. When I was gaming, the fans would be continuously on at low, but the notebook wouldn't get hot enough to kick into high (I set that around 60-65C).