1. To actually learn something that requires thinking instead of memorization.
2. To prevent stupid crap like this happening: "A Mathematical Model for the Determination of Total Area Under Glucose Tolerance and Other Metabolic Curves" (the paper he is referring to has so far gained over 200 citations according to Google Scholar)

1. Mathematics courses encourage analytical thinking in a way that may be helpful for biology majors.
2. There is some argument that calculus ought to be more widely known within the biology community. For example, see the following infamous paper, which has gained over 200 citations according to Google scholar:

M.M. Tai, A Mathematical Model for the Determination of Total Area Under Glucose Tolerance and Other Metabolic Curves. Diabetes Care, Vol 17, Issue 2, 152–154.

The "mathematical model" discussed in the trapezoid rule, which is often covered in second-semester calculus courses.