Bottom line is you need a driver when you want to switch big mosfets fast, as the gates have significant capacitance.
The driver (a) supplies the high pulse current to charge & discharge the gate quickly to keep it out of the partly-conducting state for any length of time to maximise switching efficiency, and (b) increases the drive voltage from logic level to a higher voltage (typically 5-15v) to turn the fet on as hard as possible to minimise static losses.
If you're just turning LEDs on, or PWMing at a few hundred Hz, chances are you don't need a seperate driver unless you're up in the tens of amps range.