You don't necessarily have to but if you want to backup your wallet and save it at several places (e.g. USB Sticks etc.) -- what you should definitely do by the way -- you might want to encrypt it so that it is locked just in case somebody should get their hands on your wallet.dat.
This.
You are not secure from someone physically stealing your Mac or any backup you have. Encrypt and put on several USB drives. Put the USBs in different locations.