The gate driver is essential in power electronics because of two basic reasons. First, at high frequency switching applications the switching time is less which means that in order to charge/discharge the mosfet capacitance fast, more current is required. The output current of most optocouplers is not enough to drive the mosfet properly. Secondly, most of the optocoupler output is limited to under 10 V which means that they can operate a mosfet near its threshold voltage, This results in high Rds resulting in heating the mosfet.