Well, if you're asking about other airlines too - some airlines charge you to check-in and print your Boarding Pass at the airport. Spirit is one such airline in the US. Ryanair does this over in Europe (and it's stupid expensive too). Honestly I'm not sure how it's "legal" given that they are having to provide a required document to clear security but it is so they get away with it. So in this case, even if you have bags to drop, if you're flying Spirit or another ULCC airline that charges a fee for using a kiosk at the airport, it can behoove you to check-in online for free (of course the best course of action is to avoid traveling on such airlines in the first place. )

Other reasons: upgrade offers. While no one external to DL knows the algorithm, you may get offered an UG at check-in that you consider reasonable enough to pay for and buy. Logic would dictate that checking in earlier gives you a better option as if you check-in later or at the airport, FC could be full.

For DL, another reason to begin checking-in online is via the online check-in you can initiate the Same-Day-Change process during online check-in.