UA pretty much outlined the advantages to checking in (without a printer) in the email they sent you. You're able to change seats (or see that they've moved you into purgatory while there's still time to do something about it), apply for and get an upgrade (because you're within the relevant window if one's available), and just generally have a sense of peace-of-mind that things are in order.

It also means you'll be spending less time at the airport kiosk, going through the various options.

Seems like a good thing to me!