Most user devices do not support entering a pre-hashed key. A pre-hashed key needs to be in hex and exactly 256 bits (64 hex characters) long. It is used directly in binary format in the encryption system.
Pre-hashing the key will slightly reduce startup time on devices with a slow CPU, since the hashing step can be skipped. Other than that there is no reason to do it.
The utility program wpa_passphrase can be used to hash a text passphrase.
The reason for hashing the key is to make it more difficult to brute-force guess the passphrase. In order for this measure to be effective the SSID needs to be something unique and not found in a pre-hashed "rainbow table" of common SSIDs and dictionary passphrases.