Why does a mathematics department want student evaluations of a famous professor? If a professor is very famous for his research and belongs to the most prestigious academies of science for his lifetime work, I find it unusual that the math department would ask us to evaluate him.  We did so anyway and submitted the evaluations to the director of academic affairs.

Is this typically done at most American universities, irrespective of a professor's age, status or fame?  I would think that after a certain point in a professor's career, student evaluations aren't necessary.  

Is it sort of a sanity check?  He was 85 years old.  Is it done to try and find reasons to force retirement of professors?
Being a good teacher and being a good researcher are distinct skills. If someone has a job that requires both skills, they should be being evaluated on both.