[conk]

I don't see any benefit in using an optocoupler. Using a transistor with flyback diode for protection is acceptable. Both the relay and ESP8266 are powered from the same 5V source, adding an optocoupler is not really isolating anything.

[elcritch]

Sometimes it’s easier to deal with a combined part than two parts. It can reduce assembly costs for small volumes, or if the person doing the design is just familiar with the opto.