: He could make it disappear. Tom is on a sense the opposite of Sauron. His piercing blue eye that the hobbits saw through the Ring is a strong counterpoint to Sauron's Eye. Sauron sought power, domination and order. Tom sought none of those things. He lived side by side with Old Man Willow. Tom did not destroy Old Man Willow, rather he was content to let him exist.

That's what I've always believed. If I had to answer the question, "why was Tom in the book"? I'd say he's in there to illustrate Sauron because he's everything that Sauron ISN'T. While I'm certainly not saying Tolkien intended this or that I totally believe this, it's merely if I had to answer that question.