In my own experience if you're explicit on arrival times it can also help the property plan better, e.g. if you're arriving at 7pm and let them know they may have an upgraded room available, rather than if they didn't know, so had to presume you were going to check-in at the typical 3pm and hence got a more standard room cleaned and available. Similarly, a good property would plan that you will leave at your guaranteed late check-out time, if you don't need that and make it explicit through check-in it can help with an upgrade I'd suggest as they may need that upgraded room for another guest checking in at 3pm, so if they know you'd be out well before they know its "safe" to give to you.

As such, few reasons to not check-in online, very few people do it so it puts you ahead of the queue and its more convenient when you arrive at the property and in theory everything should be ready for you straight away.