If you do the online check in, you don't get your key right away, the front desk at US hotels has to acknowledge it and approve it still (or someone in a back office at bigger hotels).

Here is why I do online check ins whenever I see that I got a room upgrade. Otherwise, it has little value to me.

I have on 2 separate occasions seen that I am getting an upgrade when I checked the app, but I didn't do the check in, and those upgrades then were taken away. Someone else booked and paid for a room or someone else booked a room and checked in before me while it was available so they got the upgrade available at check in and I snoozed and lost out. So if I see that I got upgraded to a suite, I am checking in.