Does PGP passphrase necessary if I store private key and passphrase in the same place?

I’m building a system that generates PGP key and store private key in secret vault. One thing I’m not fully understanding is the need for passphrase.

I can generate a random passphrase during the key generation and then store it in the secret vault along side with the private key, but I’m wondering if it has any benefit. If I store both passphrase and private key in the same place and that place can be considered secure, is there any additional benefit of using the passphrase? Or just storing the private key securely is enough?