I think that with most visual arts, the art springs from it’s creator’s own expression. The audience is unknown and builds based on their appreciation for the piece(s). For the graphic designer – especially one involved with marketing products, goods and services – the audience is usually known. It’s the designers task to identify their visual vocabulary and response triggers and “reverse-engineer” a piece which will cause them to act in a desired way.

So I guess, as you mention, that the “why” has a better chance of becoming evident to the viewer in as much as the designer considers their sensibilities first.

i recently talked with my seniors about the difference between “selecting an appropriate style” (borrowing from the bauhaus, or post-modern ’80s or whatever) and generating a meaningful form based on the given content, regardless of how it may end up looking. to me the difference is embedded in an honest attempt to visualize the values or perspective embedded in the content. that can, and should range from references to the written content, to the client’s values, to the designer’s convictions about how the form should respond to those first two things.

the “why” is extremely important, but it’s also very hard to make evident to viewers/readers. a lifelong pursuit in my opinion, but a lot of fun to try.