Encryption can take quite some time, memory, and network bandwidth if encrypted data is to be transported. Some applications use the same argument, or a set of arguments as one of the inputs. In such applications, it doesn't make sense to encrypt and transfer the arguments each time. Instead, arguments can be encrypted separately, and reused:
If you have multiple arguments, the encrypt method would return a tuple, and if you specify None as one of the arguments, None is placed at the same location in the resulting tuple (e.g., circuit.encrypt(a, None, b, c, None) would return (encrypted_a, None, encrypted_b, encrypted_c, None)). Each value returned by encrypt can be stored and reused anytime.
The ordering of the arguments must be kept consistent! Encrypting an x and using it as a y could result in undefined behavior.