The gate has input capacitance, so a certain current is necessary to charge it up in some time period. If you make the series resistor low it will charge and discharge faster.
Some applications you want very fast switching and others you choose to burn up some energy in the device to reduce voltage spikes caused by leakage inductances at turn off. The faster you turn of the device in this case can produce voltage spikes high enough to exceed the Vds max of the device due to the relation V=L(leakage)*di/dt
where di = change in current, dt = time to turn off
So for example with 1uH of leakage inductance and 10A current change, switching off in 100nS will cause a voltage spike of 100v over your device, which over the Vds rating for your device.
So it is a balance of the amount of energy you will allow the device to burn and the inductive spike your device can handle. With inductive loads the problem is not normally at turn on, as current will ramp up (voltage leads current) due to the inductance reducing the instantaneous power dissipation in the device
As Styx said you can have different on and off times by using another resistor and a diode in the gate circuit.