In the pkcs#10 workflow to obtain the x.509 certificate used in digital signature, in the Certificate Request Information, itself in the CSR, we have the Subject Public Key.
That Subject Public Key will be used to verify that the CSR isn't modified since the Subject wrote it. All is fine up here.
Once the verification of the CSR is done, the Certificate Authority generate the X.509 Certificate. In the PKCS#10 (3. Overview, first paragraph after the ordered list), it is stated that we give the Subject Public Key in the X.509 Certificate.
The Subject Public Key was provided because we signed the CSR with the Subject Private Key, but this private key won't be reused afterward. It might have been generated only for the CSR creation, and the CSR isn't kept in the X.509 Certificate.
Also, the X.509 Certificate is signed by the Certificate Authority with its own private key, not the Subject Private Key.
So the question is : Why keeping the Subject Public Key in the X.509 Certificate, as its goal is reached once we verified the CSR, verification done before the X.509 Certificate generation?