Of what use can a logic gate be to a circuit that contains a micro-controller
I think you mean a 'discrete' logic gate. 

Yes, there are reasons you may wish to use an external gate. I'll take an actual example: I have a microcontroller that controls a clock signal to some external boxes. It must turn the clock (several MHz) on at a time determined by the microcontroller and must not feed out any 'runt' pulses. By synchronizing the enable with the clock generator (a flip-flop) and gating it (an 'and' gate), the specifications can be met. There's no way to do it with just the micro. 

In another case, an external signal from a comparator should be enabled by a timer in the microcontroller. The microcontroller peripheral has the ability to precisely (in time) toggle its port pin, but no ability to 'and' signals with that pin state. 

In some cases, microcontrollers have been fitted with some configurable logic to take care of this sort of requirement (the 'CLC' or 'configurable logic cell') on Microchip parts, for example), but there will always be applications where the micro maker did not anticipate the application. In fact, sometimes we tie an entire FPGA with hundreds of thousands of gates to the micro to get enough external logic to meet the requirements.