It is generally recommended to safely erase the content of a disk before encrypting it, to avoid some forensic attacks.
But is it necessary when that disk was encrypted before ? Wouldn't the data left be "random" since it's all encrypted ?
Random bits of data can make up fragments or even whole files. While that doesn't happen too often, it depends on the importance of the data and will-power of individuals wanting to gain access to it. If the HDD/SSD was not wiped completely, it could contain traces of previous files/fragments/encrypted data that can be obtained. Erasing the HDD completely will remove traces of any potential files and is not considered a bad practice.
Not an expert, but I would assume that partial/broken encryption is irrecoverable even with an encryption key.