Most likely because DL charges for these bags at the gate if they don't fit the carry-on bag sizer! Or if you have too many.

I've seen this at several DL stations when I was flying them, mostly in SEA. If your bag didn't fit or if you had more than the one allowed it was being tagged to your final destination and you were giving up your credit card to have it tagged. And you weren't boarding until it was tagged. Get it?

DL rakes in far more in baggage fees because they charge for most bags to fly vs. UA who will still give it away for free at the gate (for now...)

-RM