A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 102,617
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.