1) It will move him further to the left. He doesn't have to go to the extreme that Bob Bennett is going to, but just throw the liberals a bone once in a while.
In 2006, Chris Cannon was challenged from the right flank of his party, and came out successful. However, he didn't move right far enough, and many that had sided with him in 2006 went for Jason Chaffetz in 2008.
Even the threat of a primary challenger was enough for Jon Huntsman to throw the conservatives a bone on school vouchers during the 2007 elections.
And, he has some tremendous political capital to spend.
2) He can counteract the Republicans' biggest claim against him.
Remember this from last month?
At a town hall meeting, [RNC Chair] Steele went to the heart of the matter when it comes to Matheson. The campaign theme expected next year, he said, is: A vote to re-elect Matheson is like a vote for liberal Nancy Pelosi.
All Jim Matheson would have to say is "see, the Nancy Pelosi wing of the Party hates me."