Why should do online check-in when flying?

This, from American Airlines:

"Your trip is eligible for check-in now. Save time at the airport and check- in on AA.com® any time from now up to one hour prior to your departure. To use our new instant check-in feature, simply click the button below and print your boarding pass. Save this email and use your personal check-in button for quick and easy check-in for your return flight."

Anyone have any idea what possible benefit I get for checking in online? I don't think any of these

• Save Time? How? I still have to go show my Id before getting my boarding pass don't I?

• Checked bags? Still have to go to the counter as usual, don't I?

Checking in online seems cool, but why???