Our high school orchestra director, Mr. D., never told us this directly, but students understood intuitively that if they played at least reasonably well, they'd get an A in the class.
I remember only having a music theory test once during my four years of high school orchestra, and if memory serves, I got an A-. My A grade for playing the bass well was already secure, but I still wanted to do well on the test just for my own satisfaction.
Even if it doesn't count for much grade-wise, it's enriching to have an understanding of the music you're playing beyond just getting the notes right.
