What is the theory behind a gravel bike?
GCN had a funny take on this recently: Gravel bikes exist because mountain biking is now boring. Or to give slightly more nuance, mountain bikes make it so easy to ride over difficult terrain that they remove the challenge.
Coming at it from the opposite direction, road cycling may be losing some popularity because A) roads in many places are poorly maintained due to austere public-works budgets, and B) motorists are motorists. Which creates some demand for something that you can ride on bad roads/roads that motorists don't drive on.
Gravel bikes generally are designed for a little more comfort and stability than a road-racing bike--more in line with a road &quot;endurance&quot; bike. Hence the more upright position and slacker angles. Some people ride gravel bikes with fat slicks—I don't think you can make any sweeping judgments about tires, other than they're fatter than road tires.