Just because you can do something doesn't mean you have to do it.

They offer check in up to 24 hours before a flight because they have to have some limit to it, and they picked 24 hours, and most airlines use that. It could have been 48 or it could have been 12, they picked 24.

As mentioned in the previous posts, it may come into play for upgrades and other things, but if you don't want to check in at 24 hours you don't have to, if you don't want to check in online and prefer to stand in line at the airport you can do that too.