Why would you want to use a digital envelope? As I understand it, a plain-text message is encrypted using a secret key and then the key is encrypted with the receiver's public key. The encrypted message and the encrypted key is sent as the digital envelope. Now, what I don't understand is why one just don't encrypt the plain-text message with the public key in the first place? The literature I have been reading states that the secret key used to encrypt the message is a one-time key, so wouldn't this render the digital envelope unnecessary? I would understand the purpose of a digital envelope if it was to encrypt the secret key for continued communication, but according to the literature, this does not seem to be the case.
why one just don't encrypt the plain-text message with the public key in the first place?
If you want to send more than one message, the client and server need a shared secret that they can both use to encrypt and decrypt with.
Speed is very important. Encrypting with a symmetric key is much faster than using a set of asymmetric keys when the plaintext becomes longer.