Depends really on who you are flying with. I am going to guess it isn't Ryanair - IF IT IS then check in as otherwise they will charge you to check in at the airport (their desks are now called baggage drop).
If it is Aer Lingus, the main advantage I think is getting to choose your seat, leave it to when you arrive at the airport and if it is a popular flight you could well end up in a middle seat or a selection. You don't have to print your boarding card as you can do that at the machines in the airport.
For others, it depends but usually there is a value in checking in as early as you can and Ryanair aside they are accommodating about giving you the boarding card at the airport.