[ollien]

It's been a few years since I applied to college, but subject tests were nice because they showed a baseline for certain subjects that didn't really have AP exams (e.g. Math II is everything through pre-calculus). You can also take them more than once, unlike AP exams. Also, you don't send "official" AP scores to college admissions; you only do that once you're admitted, since they actually go to the registrar, not admissions. You simply self-report AP scores otherwise.

I feel like there are other benefits that I'm forgetting, but those come to mind immediately.