Here are some:
*Help negotiating advanced classes (one DS scored 24 of 25 on math in 5th grade which supported my argument that he was past middle school math).
*Experience with test taking--getting comfortable and start learning test taking strategy. High scores on ACT/SAT equate to real $ in scholarships. They figure out how to budget time, guess when it's ok, leave blank when guessing is not advised, etc. Whether we're fans of testing or not, a lot is based on certain tests.
*Fun - Some kids (not me, but definitely my boys) think it's fun to work out above level test questions. This was a great "enrichment" suggestion from the school district when I called to say my oldest was bored through and through. Their one suggestion was above level testing...for fun, challenge, whatever. Up till this point, oldest DS had only taken annual tests with grade-mates. not much is gained when he topped those. This raises ceiling and shows closer to true ability.