What's the purpose of this RC filter?
The circuit is the following and i think this RC filter is used for anti debounce purposes but I'm not sure. 


Yes, debounce but it could also filter / absorb any noise or spikes on the incoming line as the circuit is a low-pass filter.


  Is it to prevent the optocoupler and the indicator LED from being "damaged" by the debounce effect of the switch?


More likely just to debounce.


  And the circuit doesn't connect to any microcontroller so is that RC filter really necessary in this case? 


It isn't directly connected to a micro-controller but presumably the opto-transistor is. Without the debounce circuit the opto-isolator would pass the contact bounce through to the following circuit giving multiple pulses. Presumably this would be a "bad thing" so they have prevented it.