Optoisolators (also known as optocouplers, or just optos for short) are a valuable tool for passing signals across isolation boundaries, but all too often one sees designs in the arduino/pi/maker world that use an optoisolator but do not place it across an isolation boundary.
A common example is driving relays. Lets say the relay contacts are used to switch dangerous voltages. Naively one might think that two isolation barriers are better than one, and therefore using an optocoupler to drive their relay will make things safer.
The thing is the relay coil needs power! So unless you arrange a separate power source for the relay coil, which is isolated from both the "safe" and "dangerous" parts of your system, you have nullified one of your two isolation barriers. Most common in opto+relay setups is to see the opto nullified, while the relay provides the only effective isolation barrier.
Now relays can be perfectly good isolation barriers, but your focus should be on firstly selecting an appropriate relay and secondly laying out the PCB in a way that maximises the isolation. Not on adding an optoisolator that gives you a false sense of safety.
It makes me cringe when I see a relay board advertised with ineffective optoisolators "for safety", while completely screwing up the creepage/clearance around the relays that form the only effective isolation barrier.