You can see the difference between driving a real mosfet with or without a gate resistance. Closing the switch over the 15ohm resistance bypasses it: the parasitic inductance of the wire bonds and the package itself causes a lot of voltage noise on the pins, causing a fast on-off sequence at every change of gate input. Increasing the gate resistance will slower the rising edge (goes up exponentially), reducing the switching noise and increasing stability and temperature efficiencies. If the resistance is too small, you'll have a bad switching performance due to the oscillations... If it's too big, the time to turn it on will be so big that it won't work anymore at high frequencies.
The two-way switch on the left gives the signal to the gate.
