I was reading the XML Encryption standard and I have some trouble understanding the purpose of encrypting some plain text with a symmetric generated AES or 3DES key that in turn gets encrypted with the public RSA key of the recipient.
If an attacker gets the private key of the recipient and has the network traffic recorded then he or she can decrypt the AES key and then the plaintext encrypted with the AES key. In TLS or in Signal the symmetric key gets negotiated with DH and does not travel encrypted over the wire giving the connection forward secrecy.
Why should I encrypt the symmetric key with the asymmetric one? Does increase confidentiality or performance? When should I use this double encryption?