At the beginning of Episode IV, Luke and Uncle Owen purchase C-3PO and R2-D2 from the Jawas. Well, whenever Uncle Owen talks with 3PO he mentions that he needs a droid that can understand the binary language of moisture vaporators (along with Bocce), but it's never mentioned what purpose R2 would serve...
Why would a Tatooine moisture farmer be in need of an astromech droid?