This question came to me when I saw this question driving a relay with transistor and Opto-isolator.
As Olin Lathrop had pointed out in his answer, relays already provide isolation between the coil side and the switches. Then any reasons to isolate the MCU from the relay driving transistor?
The only reason to further isolate mcu and relay coil is if you have yet a different isolated PSU that drives the coil, otherwise I see no explanation. Items are sold on ebay and people wire all kind of stuff together, therefore I presume the sellers introduced this feature to prevent MCU boards to burn.
Of course the singnal from MCU needs to be buffered (transistor) in order to drive a relay coil.
Another possibility comes when you have opto isolated inputs. Let we have a PSU for MCU and other low voltage periherals and we want opto isolated input/outputs 24VDC. A second one PSU 24VDC is needed for supplying inputs, with same 24VDC you can drive a relay trough opto isolated output, so the MCU PSU is relatively small power, now it makes even more sense to have opto isolated relay.
In general, no.
But when the relay coil requires a large current it might be good to keep that current (or rather the current chnages) out of the uC circuit. Optical isolation could be one way to do this.
