Why does Atticus ask whether anyone has called a doctor and why is this important?

There are several reasons Atticus asks about the doctor. First, Mayella's father has testified that he interrupted Tom while Tom was raping Mayella. A doctor could provide physical evidence of such a rape.

Also, the father's failure to call a doctor demonstrates forcefully that his primary interest in accusing Tom is to protect his family's reputation, such as it is, rather than concern for Mayella's welfare. This is in sharp contrast to how Atticus treats Mayella when he questions her; he shows genuine tenderness toward her. He understands how difficult her life is, and why she has been forced into the situation she is in.

In order to assure that she would be alone with Tom, Mayella, who is usually responsible for her many younger siblings, carefully saves money for many months so she can send them all to buy ice cream.