Question: As I understand a gravel bike, it's basically a road frame but perhaps with a slightly more upright riding position. It has wider tires with much deeper treads and a stiffer rubber than standard road bike tires. What is the theory behind these changes? I had always thought that the thicker and stiffer tires were to prevent small bits of gravel from pinching flats, but is the theory instead that the deeper treads and wider tires are supposed to increase traction? And what is the rationale for the upright riding position?

Answer: 
What is called a "gravel bike" is really a new word for a cyclocross bike, but with maybe a tad less aggressive geometry. And cyclocross goes back pretty much to the beginning of bicycling, as the first races were held on unpaved courses and no roads were paved yet, or at best cobbled. The original road bikes looked a lot more like today's cyclocross bikes, only a pound or three heavier.

I personally don't like the name "gravel bike". Gravel is the stuff they dump along the train tracks near my house to deter mountain bikers, or the stuff you decorate your yard with if you live in places like Las Vegas, but I rarely encounter gravel out on the trails here in northern California. Maybe some gravely stuff mixed in with loose dirt intermittently, but there's nothing too fun or challenging about riding on a layer of gravel as there is a natural dirt or muddy path.

In theory, I doubt you will be able to ride one off road without encountering mostly dirt. The style of bike is ideally suited for trails. They should call them "fire road bikes" or something more realistic if "cyclcocross bike" is too worn out for the marketers.

I would call the bike I commute to work on a gravel bike because I built it with a steeper stem to bring the bar up so I can see traffic easier, and a single, low gear, 38/18t. But it started life as a 'cross bike. A gravel bike might be ideal as a commuter bike, especially if you can outfit it with panniers, lights, etc.
There's an interesting discussion with [Josh Poertner](https://www.youtube.com/watch?v=asEYkpW0vwQ), an aerospace/automotive engineer turned pro cycling consultant, on finding the most efficient tire pressure for different riding conditions. Basically, high pressure tires are less efficient on imperfect surfaces because of kinetic energy lost by bouncing the whole bike up and over a small object or road crack, where a low-pressure tire can absorb most of that impact without disrupting your momentum. The rougher the riding surface, the lower the pressure needed for optimal efficiency. Wider tires can run at the lower pressures needed for efficient riding on gravel, and the aero tradeoffs are smaller at gravel speeds.

I think the more upright riding position has to do with conservation of effort and a reduced tradeoff between aerodynamics and handling. Since your not breaking any land speed records on gravel, getting max aero out of your stance isn't as crucial, but picking (and staying on) the best line often is. Raising the bars up lets a rider use flared-out drops to selectively widen their grip for better control, when needed, without getting completly horizontal in the process. I've seen some gravel riders who spend upwards of 50% of their time in the drops, vs. road cyclists who are maybe using them 10-15% of the time.

These are just some of my observations as a strictly non-competative cyclist who only recently got into gravel. I also happen to think my basic-B of a gravel bike makes the perfect everybike for all around riding. It's a great all-weather commuter, has almost identical gemoetry to some high-end randonneuring frames, and can even tackle a bit of singletrack.