For a typical Biology Department, TAs are a critical resource. Teaching Assistants run most of the labs in the department, and in some cases run recitations or help grade exams in large classes. In most places I have been the TAs are limited in the number of hours they can work in a given week, usually in the range of 20h/wk. TA support comes with pay that covers the stipend, and importantly, the tuition and fringe of the student for the semesters they are teaching. In that sense, their major professor does not need to support them off grants while they TA, but their research time is limited by the contact hours, lab prep and grading.
Since many biology departments are largely geared towards NSF funding, TA support allows for more students to be involved in a project than can be supported directly from a single grant. In my college, for instance, the Dean's office will match a semester of TA support for every semester or RA support a PI has on a grant. This allow us to be a little flexible in our budgeting, since the actually dollar amount of federal grants has not climbed appreciably in quite some time, whereas inflation and institutional overhead rates (which IS counted into the budget of an NSF grant) have increased, unabated.
Therefore, we have graduate students performing an important role in the teaching mission of a department as a way to directly supplement the research mission of the department.
And this is where it can get tricky, folks. Because not every class runs the same way and not every professor understands the big picture. If you think of TAs as graduate students who are teaching to supplement their research time, you will have very different expectations than if you see a TA as a junior teacher there to relieve teaching burden from the professor. There will be different task and time expectations and the inequity of these across the curriculum can be significant. As graduate students, it's important to know what the expectations are when you agree to take on a new class.
But more importantly, departments need to ensure that there are cultural norms for these expectations. Is it expected that TAs should work their full time allotment every week? If not, what is a reasonable load? After all, the grad students are there to get their degree, not bear the burden of your teaching load.
EDIT: I forgot an important point that I was reminded of on Twitter: TA's are paid for at the university level by overhead dollars. Thus they are paid for by research to support the research mission.