"where is this 'noise' coming from exactly?"
Most of your noise would be comming from the long RS232 leads, if you use rs485/422 the signal is differential and the noise cancels each other out.
opto couplers remove ground noise which means if you have a long signal ground the ground voltage might not be the same in different places(not 0v everywhere) then you get problems and might damage your pc.
Optocouplers solve this problem and protect your pc also.
best regards
Mahmood