What is the purpose of the optocoupler in this circuit to drive the mosfet/relay?
The schematic is badly laid out. The earth symbol is upside down, current is flowing from bottom to top and there is an unnecessary break in the RELAY net. (By mirroring the MOSFET and R2 the opto-transistor emitter could be directly connected to the gate.)


  ... the relay is using one common GND and 3.3V Vcc, so why isolating the control signal?


Many of those common hobby relay boards feature jumpers which allow the board to use one supply for everything (as shown) or the 3.3 V supply for the opto-LED only and a separate supply for the opto-transistor, FET and relay.


  Do I miss something, why the optocoupler is still a good idea?


The isolation is there if you chose to use it.