There are many types of museums. They collect artifacts, objects and stuff from individuals, artists, both cultural and items of historical significance. They are a testament to a moment in history. A mill is an artifact within its self. A mill may contain evidence and records of various moments in its past.
Mills too can also be museums, either they are static or operating.
If a particular mill was water powered, they contain a variety of things including sources of power. The building contains material for manufacturing. When many mills stop operating much of the original machinery may have been removed. Now that they are open for the public to view, they pass on more information than just the products of their operation. In America the miller may be a retired person, with a passion for what he does.