This question already has an answer here:
I always thought that the point of "checking in" to a flight was so that the airline knew you were at the airport and would most likely board the plane.
But nowadays you can check in by computer or smartphone, which is great -- but you can do so as much as 24 hours before the flight. That's before you even go to bed the night before the flight, let alone arrive at the airport.
If there's a emergency, or even very bad traffic, that makes you miss your flight, won't it be in the last few hours? What good is checking in so far in advance?
EDIT: And almost as soon as I finished typing my question, I saw this: What is the purpose of check-in before flying?