This has been going on a while, possibly as a test for some of that time (I recall hearing it in PHX for the first time about a year ago), but I have not heard it on shuttle flights.

I'm not sure if it's to speed boarding or to encourage bag or gate checking, but I'm amazed at the people who get excited to take advantage of this offer. If I don't need overhead space, my goal is to be the last one on before the door closes.