Chagrin:
It's the inductive spike from the relay coils that you're isolating from. I'll agree that the inductive spike is usually not a problem (as long as the module is properly designed with a flyback diode -- and it is); I'm just trying to set the record straight. OP never provided any detail about his project that would let us know if the optical isolation was appropriate or not.The "VCC" pin on the relay module is only providing current to the LED side of the optocouplers. The "JD-VCC" pin is providing current to the transistor, optocoupler phototransistor, and the relay coils. There is appropriate isolation in that respect.
They certainly can be set up to offer 'double isolation' if wired up properly, just overkill for many projects/applications in my opinion as most cases one would want to utilize the arduino's 5V pin to power the coils rather then the added expense to use an independent voltage source just for the relay coil power . Not a bad thing that they are 'over designed' in that matter as the price is very attractive and you can set them up anyway you wish. I've bought a few and they work fine. I've always questioned if the relay quality was up to pare with US manufactures but I've no way to determine that.