Relays already provide galvanic isolation, so adding an optocoupler between a relay coil and an MCU is redundant in that respect—that’s what I suspect tristan was getting at. There are cheaper, more compact ways of dealing with the other issues you mention, especially on an integrated PCB like this.
In this case, it looks like they’ve got a couple shift registers on that little black interface board driving optocouplers which in turn drive some transistor arrays that finally drive the relay coils. That should give you the ability to run several of these boards from mutually isolated DC (coil drive) supplies, which I guess could be handy in a few cases, but I’m having a hard time thinking of any.