Originally Posted by
MSPeconomist
If there's some problem with your ticket or reservation, it's better to have one day rather than one hour to fix it before missing the flight.
Depending on the carrier's rules, checking in early gives you priority for upgrades or avoiding IDBs or involuntary downgrades. In some cases, you must check in to see your position on the upgrade list.
This is probably the main reason overall. I will also add that not also checking in, but having a paper printed BP is also good to have, to minimize the fumbling around with the mobile BP. Also some security kiosk sometimes have problems reading the mobile BP.