Question

. Why do carpenters use a screw instead of nails to join two pieces of wood?

Answer 1

Screws have phenomenal tensile strength which is far greater than nails. Screws build a tight adhesion between two pieces of wood.  

Nails are used for joints, because even after multiple twists the wood at the joints will remain together when nails are used. But a screw will come off after a single twist, if screws are used to hold wood at the joints.  

For any static wood work screws are better than nails because of their tensile strength.  

Answer 2

Carpenters use nails instead of screws. If that isnt the answer you are looking for than another answer would be that screws hold better than nails. When you screw wood together they don't slide or turn sideways like when you nail wood together. When they are nailed together you can turn the two pieces of wood while they are stuck.