If you need to request a certificate to use for a website, you need to generate a certificate request resulting in a certreq.txt you need for buying the actual certificate. Once the certreq.txt is generated, you can not use another certificate for your website because your request is still pending. The below figure shows the dialog you’ll see when trying to setup secure connections for a website for which a certificate request is pending:

There can be days, even weeks between the actual request for buying a certificate and the moment it is delivered, which voids SSL-communications for your website. To avoid this, you can create a new website, then use a development certificate and use that for your site until your actual certificate is delivered.

If your actual certificate is delivered, you need to delete your temporary website and you can use the actual certificate.