I recall that I chose to take these because Stanford recommended the MATH II at the time of my application. This is no longer the case. Stanford only requires the SAT or ACT these days. However, these are also pretty straightforward exams to study for and they usually match up with the lessons that you are taking in local high schools during senior year.
Important Note: You don’t have to take both Math Subject Tests. You can take Math II without taking Math I.