It's not really a mistake. In most cases, the optocouplers may work in the design just fine and there are no mistakes in the design, but it's just a component that's useless, so why put useless components in the design for no reason.

It's a specific instance of "carco cult", a thing used or added because everyone else seems to use it for some reason that nobody really seems to know why exactly.

It's so cool to be able to say your design has an optocoupler as it is generally used for safety and isolation, but in reality, in most Arduino designs, it is usually designed in a way that does not isolate anything, wastes power to do it's function of passing signals through it and just adds cost and PCB size to the design, as the design could even be just done without the useless optocoupler, maybe even better.

Like in the post you refer, the optocouplers add no value. They don't isolate. Driving the optocoupler input draws more current from Arduino GPIO pin than driving the transistor base directly. Optocoupler output leakage current may even cause problems with the transistor not being able to fully turn off. The transistor is anyway required as the Arduino or optocoupler output cannot drive the current needed by the relay.

It's not really a mistake. In most cases, the optocouplers may work in the design just fine and there are no mistakes in the design, but it's just a component that's useless, so why put useless components in the design for no reason.

It's a specific instance of "cargo cult", a thing used or added because everyone else seems to use it for some reason that nobody really seems to know why exactly.

It's so cool to be able to say your design has an optocoupler as it is generally used for safety and isolation, but in reality, in most Arduino designs, it is usually designed in a way that does not isolate anything, wastes power to do its function of passing signals through it and just adds cost and PCB size to the design, as the design could even be just done without the useless optocoupler, maybe even better.

Like in the post you refer, the optocouplers add no value. They don't isolate. Driving the optocoupler input draws more current from Arduino GPIO pin than driving the transistor base directly. Optocoupler output leakage current may even cause problems with the transistor not being able to fully turn off. The transistor is anyway required as the Arduino or optocoupler output cannot drive the current needed by the relay.

It's a specific instance of "carco cult", a thing used or added because everyone else seems to use it for some reason that nobody really seems to know why exactly.

It's so cool to be able to say your design has an optocoupler as it is generally used for safety and isolation, but in reality, in most Arduino designs, it is usually designed in a way that does not isolate anything, wastes power to do it's function of passing signals through it and just adds cost and PCB size to the design, as the design could even be just done without the useless optocoupler, maybe even better.

It's not really a mistake. In most cases, the optocouplers may work in the design just fine and there are no mistakes in the design, but it's just a component that's useless, so why put useless components in the design for no reason.

It's a specific instance of "carco cult", a thing used or added because everyone else seems to use it for some reason that nobody really seems to know why exactly.

It's so cool to be able to say your design has an optocoupler as it is generally used for safety and isolation, but in reality, in most Arduino designs, it is usually designed in a way that does not isolate anything, wastes power to do it's function of passing signals through it and just adds cost and PCB size to the design, as the design could even be just done without the useless optocoupler, maybe even better.

Like in the post you refer, the optocouplers add no value. They don't isolate. Driving the optocoupler input draws more current from Arduino GPIO pin than driving the transistor base directly. Optocoupler output leakage current may even cause problems with the transistor not being able to fully turn off. The transistor is anyway required as the Arduino or optocoupler output cannot drive the current needed by the relay.