When airlines started implementing fees for checked luggage, carry-on luggage was exempt. However, if you brought with you a carry on bag that was too large for the overhead bins, your bag was checked at the gate and stored underneath the plane with all of the other checked-at-the-counter luggage.

So naturally, everyone started using carry-on luggage that was slightly too big for the overhead bins. Not only did you get your bags checked for free at the gate, your bags were returned to you at your arrival gate and you didn't even have to endure the hassle of going to baggage claim.

Additionally, this also caused flight delays since gate personnel/flight attendants had to tag, store and retrieve long lines of luggage at the gate, causing passengers to miss connecting flights, etc.