USA Today has the highest circulation in traditional newspapers. Not really sure why that is, but my guess would be that they have some enduring relationships with hotel/motel chains. It's definitely a brand that resonates.
USA Today was the first newspaper of its kind. Flashier, more concise, graphic-laden news. A larger focus on celebrity, entertainment, etc. news. From wikipedia on USA Today: "derided by critics, who referred to it as "McPaper" or "television you can wrap fish in," ". It was the Buzzfeed of its time.
It's a generic newspaper without a regional focus, unlike the NY Times, LA Times, etc. It's distributed nationally, and it's popular with travelers and companies who cater to travelers (airlines, airport kiosks, hotels, etc).
Our city's paper focuses exclusively on local and regional issues and includes a USA Today section to cover national stories. I suspect a lot of smaller metros' papers do the same.
Smaller/local papers also make extra money on the side through print contracts -- USA Today pays the local paper to print USA Today for distribution in that area.
Thirty years ago, I traveled a lot for work. In hotel rooms in Springfield, Missouri, or Dubuque, Iowa, I would often enough find USA Today slid under the door when I got up. Now, USA Today didn't really stack up against the local papers in Chicago, New York, or Atlanta, but a smaller city's paper could be pretty thin, and with national coverage largely pulled from the wire services.
I tend not to read it much, but it would serve for breakfast reading in a small town.