Answer:
To emphasize the Atticus's empathetic nature, his respect and consideration towards everyone, which later on serves to save him from a difficult situation.
Step-by-step explanation:
Atticus discusses crops with Walter Cunningham because he knows that it is a topic Walter would understand, Walter only goes to school because he has to and not because he wants to, so asking him about school would have been pointless.
Atticus puts himself in Walter Cunnigham's shoes and treats him respectfully, talking to him about something he actually understand to show that he respects him no matter where he comes from.
This respectful treatment of others would save him later on from a mob lynching lead by Walter's father.