Symphonic "filler"

In most major symphonic works you will hear certain sections that cause you to wonder, "Why is that in there; what purpose does that serve?" It sounds like the composer had his piece set up, and knew what he was going to do for a big finish, but he needed something to stick in the middle to sort of round out the movement. I call it "filler." The really great works do not have much of that sort of thing. Slow movements were one place where composers are hard-pressed to avoid it; it is hard to keep slow music really interesting. However, some have done it, with outstanding results.