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: 
My theory and it is just a theory. In the U.S. at least there has been a proliferation of unpaved bike trails due to the conversion of abandoned railroad rights of way to bike paths. They are for the most part either flat or have a slight grade. Most of these trails are too uneven for a conventional road bike. At the same time they are not much of a challenge for a mountain bike and are mostly an access point to more technical trails. So now we have a reason to buy yet another type of bike. With a gravel bike the gearing allows much higher speed than on a mountain bike. You're on a route that is pretty much guaranteed to be free of motor vehicle traffic so you can basically go as fast as you're comfortable riding. The larger tires allow better traction on loose surfaces while increasing comfort. So while the riding environment is similar to cyclocross the two frame geometries are different in a sense like a Tri bike and a road bike, functionally the same but tweeked for a more specific type of riding.
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.