Online check-in is one alternative to traditional check-in at the airport’s check-in counters. If it is possible to check in online, that is usually preferable. Some airlines charge extra if you check in at the airport.
When you check in online from home, you're already one step ahead in the process before you arrive at the airport. If you travel with only hand baggage, you can go straight to the security checkpoint when you get to the airport.
Please note that even if you check in online from home, you must check in your baggage at the airport.