Why use RSA-2048 for plaintext when AES-256 is much more secure?
AES is symmetric encryption.  Both sides must have the same secret key.
RSA is asymmetric encryption.  Each side has a public and private key.  You share your public key so that others can encrypt a message such that only you can read it (with your private key)

There are situations where the key exchange for AES isn't reasonable.  For those situations, we use asymmetric encryption methods like RSA.