It seems like a silly waste of time to ask us why: the people in this forum aren't the people who require it, so it's not clear what kind of answer or insight you hope to get.
But if you're just looking for speculation, I suspect it's for a few reasons. First, some people do care about the verbal score. Pete Clark says somewhere on Math StackExchange, for example, that he looks for decent verbal scores. Second, it is usually a general requirement of the graduate school, not the mathematics department. That can be for several reasons. In some cases it is considered in the financial award: higher scores can elicit better financial packages. It also provides a standardized measure of ability that controls for variation in grading from school to school, and from teacher to teacher. An A from school X need not mean the same thing as an A from school Y, but a 165 is a 165.
This is not to say I think the GRE should be required. But this is the kind of response I bet you would get if you asked someone in charge of these things.