One thing you don't have much control over in your Mac is when and how the cooling fans start. They're automatically controlled by your CPU.
However, smcFanControl lets the you set the minimum speed of the built in fans so that you can actually increase the minimum velocity to make your Intel Mac run cooler. Because this can be dangerous, smcFanControl doesn't allow you to go under a minimum speed (basically Apple's default) so that you don't risk burning-out your Mac!
smcFanControl also doesn't override the automatic settings in your Mac so that your fans will kick-in when the CPU needs them. Apart from this, smcFanControl also displays the temperature and fan speed in the menu bar, lets you set different minimum speeds for every fan separately and alters them accordingly when the power source changes.
Be warned though that messing around with your fan settings is to be done with extreme caution. Unless you work in a particularly hot environment, or feel that your fans are actually on too much for some reason, I'd be very careful before using smcFanControl.
