I've found minimal to zero benefit of using online checkin. Honestly, the only thing that has any use is being able to chat/send a message to the hotel... and I think you can do in the app without actually checking in also. And of course that assumes the hotel actually checks/looks at messages and replies in a timely manner and not hours later.

Whats really stupid is some properties that have online/mobile check in but still have magnetic key cards that you insert into the door, so its impossible to get an electronic key anyway.