The benefit as explained by others is that you don't need to get your passport verified either at the kiosk or by staff if you have checked in online. If you checked in online you still needed to use the kiosk when you turned up at the airport just to scan your passport, or give your passport to ground staff at check-in or the gate or give it to lounge staff to get this done. I never bothered with online check-in for TT flights as there were zero benefit in doing this when you still needed to rock up to a kiosk at the airport when you arrived.

Honestly it amazes me every time I sit in the lounge in WLG and keep hearing "Mr/Mrs/Ms X please come up to the counter with your passport". All these people who don't pay any attention to what it says during the OCLI process.