Where is a statement of the purpose of physics.stackexchange?
The tour (available from the big question-mark symbol at top right) states the following:
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics and astronomy. It's built and run by you as part of the Stack Exchange network of Q&A sites. With your help, we're working together to build a library of detailed answers to every question about physics, astronomy and astrophysics.
This is the statement of purpose of the site.
The original purpose for the physics SE was to furnish a place where professional practitioners of the art could get assistance from their peers in solving specific technical problems in their field. Over the years it seems to have also become a place where lay people can pose questions about physics to get help understanding it across a fairly broad range of sub-topics within the field.
Whether or not this evolution in purpose is a good thing has been a topic of debate for as long as I have lurked here (~8 years and counting).