That is where the real science occurs. It also gives you a home base in an otherwise large institution, not just a locker and a student number. It is as simple as that. If money is not motivation enough, then think about this. Mathematics is the sharpest tool of rational thought.
Biological problems seem sooner or later to yield to mathematical formulations.
Most theoretical biologists, though not all, have a substantial background and ability in mathematics. There is a subtle difference between mathematical biologists and theoretical biologists. Mathematical biologists tend to be employed in mathematical departments and to be a bit more interested in math inspired by biology than in the biological problems themselves, and vice versa. Mathematical maturity ranges widely, from those who found a need for mathematics late in their careers and were forced by the problems they tackled to train themselves in mathematics, to those who loved math, or physics, or computing before anything else and found ways to apply this love to biological problems. Excellent places to learn math, in addition to the obvious departments, are in physics, chemistry, engineering, computer science, statistics and business courses.