Progress comes from conflict. Evolution is an example of this, predator/prey relationships. Cheetahs evolve to become faster and faster, gazelles evolve to have higher endurance and agility. As each improves, the other must catch up to survive.
This dynamic exists in sports as well. In football, defensive tactics evolve to counter offensive tactics from season to season. Offensive tactics must iterate in response.
I think there's can be oscillation between marketing and creating. Create a product and try to sell it. The audience's response can inform the next iteration of the project.
On a personal level, learning is oscillation between theory and practice. You learn a concept, then put it into practice. Coming up with ideas and then testing them in the real world. Using the results to come up with more ideas.
Using only theory doesn't expose you to how your theory will breakdown in the real world. There are always edge cases.
Using only practice leads to local minimums and incremental gains. There won't be any deviation from the way things have always been done.
The scientific method is a formalized approach to this concept. Propose a theory, construct an experiment to test the theory, compare results against the theory, revise theory and repeat.
This is why the scientific method works. It's the implementation of oscillating between theory and experiment.
Given this, what is an expert? An expert is someone who knows more on a specific subject than almost anyone else. To get to this point, the expert must have learned and thus must have oscillated between theory and practice. Therefore you can see if someone is not an expert if they do not practice what they preach. Example, there is no such thing as an unhealthy expert on health. That's a sign of someone who's "expertise" comes from theory.