If music (and art in general) were just about getting political messages across, both Bob and Mark should have become politicians. I think it's much more about the sharing of feelings, though. By writing a song, you're communicating: This is how I'm feeling about this or that. With any luck, other people will feel the same, and that's rewarding for both the author and the audience. Both realize that they're not alone in how they feel about the world. That,I think, is about the best thing art can achieve.
Oh, and Dusty, you must have been kidding. Of course Mark Knopfler is a master of his craft. Why else would you spend so much time on a forum dedicated to him, and even found the forum in the first place?