A Certificate Signing Request (CSR) is required as part of the process of obtaining an SSL/TLS certificate for a website or server because it provides the certificate authority (CA) with important information about the applicant and the domain(s) for which the certificate will be used. This information is necessary for the CA to generate a certificate that can be used to establish secure, encrypted connections between the website or server and its clients.
The purpose of the SSL/TLS certificate is to provide clients with assurance that the website or server they are communicating with is legitimate and that their communication is protected by encryption. In order to provide this assurance, the CA must verify the identity of the applicant and the ownership of the domain(s) for which the certificate will be used.
The CSR provides the CA with the information needed to perform this verification. The CSR typically includes the public key of the applicant, the common name (the fully-qualified domain name), and information about the organization that operates the website or server. The CA uses this information to verify the identity of the applicant and the domain ownership, and if everything checks out, they will generate a certificate that includes the public key, the identity information, and a digital signature from the CA.
The digital signature from the CA serves as proof that the certificate was issued by a trusted authority and that the information it contains is accurate. This is important because clients will rely on the certificate to determine whether or not to trust the website or server they are communicating with.
In summary, the Certificate Signing Request is required because it provides the CA with the information necessary to verify the identity of the applicant and the ownership of the domain(s) for which the certificate will be used. This information is necessary to generate a certificate that can be trusted by clients and used to establish secure, encrypted connections.