Yes. They make an announcement asking for anyone willing to check their bag at the gate rather than carrying it on, and anyone who does so gets priority boarding (at the same time as the "families with small children and anyone who needs more time to board")
I've never flown Alaskan, but it's pretty obvious what the appeal is: it's a small optimization to make flying a little more convenient and enjoyable for some people. It seems like it would be nice not to have to wait in the boarding line and manhandle my carry-on. Plus, the policy creates some goodwill because they're passing up an opportunity to nickle-and-dime you.