Spare the obvious WN where your boarding position is based on check in and the tickets that you are unable to select your seat on, is there any actual benefit to checking in ASAP? Some airlines automatically check you in 24 hours before your flight (NH). Is the benefit for the customer or the airline? I'm taking a wild guess and thinking that it probably helps the airline know if a seat will be vacant or not and will let them reassign your seat to someone else on the standby list, but other than that I personally cannot see a difference in checking in 24 hours online or 2 hours before my flight at the ticket counter. I see a benefit in getting my boarding pass online/mobile if I don't have any luggage to check, but I usually get a printed boarding pass when I drop my luggage anyways and sometimes I don't have internet access to get a boarding pass.

It depends on the airline, but on some carriers if they have to resort to IDB the time checked in can be the deciding factor.

But otherwise, I often check in within 2 hours of departure.

Always wonder that myself as even if you check in online you can still be a no show.

I never check in online.

On some airlines, you can check in early and pick the seat you want.
On other airlines, you can get a mobile boarding pass, which means 10-15 mins saved trying to check in at the airport.

But the biggest advantage for me is the peace of mind that if I'm able to check in, then there's nothing wrong with my flight booking or ticketing.

I guess this is the main benefit consumer wise, other than selecting a seat if you were previously unable to do so. Above all else I'm guessing its mostly a money saving thing for the airline, since if you check in online you'll most likely get your boarding pass at the same time and they don't have to print it for you.

I check in as early as I can because people change flights, seats, etc. and that way I can pick a better seat if one is available. I also like to print my boarding pass when I get to the airport. I always like having a physical BP. On United I can pick an Economy Plus seat for free at check in so there is that incentive.

Well, in some cases it may allow you to choose a better seat (free'd up by elites who just got upgraded), and that seat may gone by 2 hours out.

Also, if you qualify for PreCheck, and the airline supports it, remember it's not guaranteed, so you want to know sooner rather than later if you didn't get PreCheck on this boarding pass so you can allow more time for getting through security. Finding that out 2 hours before the flights may be too late, given recent non-PreCheck security lines at some airports .

Or if you checking in online at an airline where you've not done that before (or not in a long time), you might want to know sooner rather than later if you're going to have technical problems with it (and thus may need to allow time to check in at the airport).

So while checking in exactly 24 hours out may not make much difference, checking in as soon as is convenient after that (rather than waiting until it's 2 hours out) would seem more prudent to me. 2 hours out requires everything to work perfectly, which doesn't always happen.

If there's some problem with your ticket or reservation, it's better to have one day rather than one hour to fix it before missing the flight.

Depending on the carrier's rules, checking in early gives you priority for upgrades or avoiding IDBs or involuntary downgrades. In some cases, you must check in to see your position on the upgrade list.

I like to have the printed (or maybe mobile) boarding pass in my hands just for my own peace of mind. So I will online or mobile check in even on carriers that don't require it.

This is probably the main reason overall. I will also add that not also checking in, but having a paper printed BP is also good to have, to minimize the fumbling around with the mobile BP. Also some security kiosk sometimes have problems reading the mobile BP.

That is the mai reason in my opinion, though I understand the complain

As far as I know, when you check in online, there's no such thing as a no show. You checked in so it means no more rebooking, ticket is used already. I always check in online, I don't have to queue.

No showing has nothing to do with checking in online. There may be standbys or your airport may have a time conformance system whereby you need to be past a certain point so many minutes prior to the flight. And of course the airline can still move you to another seat for any number of reasons.

online = dont have to at airport (with carry-on only)

The #1 reason for online check in for me (as you say, when not checking bags) is that if unexpected traffic etc makes you miss the airport check in cutoff time, you can still get on the flight. But for this, it doesn't matter whether you check in right at T-24, or wait until an hour before while sitting in traffic en route to the airport.

One reason to check in ASAP that hasn't been mentioned:
Some airlines (e.g. AA) use check in time as the tiebreak for standby lists. Even if you aren't planning to standby, flight cancellations or missed connections may leave you on a standby list for a full alternate flight, and an earlier original check in may be the difference between getting that last seat or not.