Back in the day, before the internet, everybody had to check-in at the airport. The purpose was to confirm that the ticket holder had actually turned up on time, and their baggage got processed. Nowadays, it's not really a check-in; for some airlines it's a thinly veiled excuse to get the punter to have the hassle of printing a ticket (and the cost), and little else.
However, the european Denied Boarding regs do make check-in an important key stage as to whether compensation is due or not in the event of delay/cancellation. The way I read it is that if I've checked in 2 weeks in advance, and the flight is majorly delayed/cancelled, I can legitimately claim compensation, even if I never turn up at the airport.