Southwest gives you a form on arrival with info to fill it out online for your return. Several people didn't do this so checking in took longer waiting for people to fill out the form.

I just returned from St. Maarten on American. At check in to leave US, they asked us to download VeriFLY. When we were returning to the US, we were just asked the questions at the airport and then we had to sign it so we never used VeriFLY