I have seen it various times on different products when configuring a SSL certificate for the web gui, where it's written that the Private Key is optional. Isn't that client (borwser) sending data encrypted with the servers public key and the server verifies (decrypts) with it's private key ?
I have no idea what the GUI you've shown is exactly for. But, for a server certificate in HTTPS the private key is always needed. Contrary to your statement the private key is not used by the server to decrypt client data (only kind of when RSA key exchange is used) but to sign a challenge based on client data in order to authenticate itself.
When a new CSR is generated to be signed by a CA, you have the option to use the old private key or generate a new one. I expect that is what this is for.
Generally, it is good practice to use a new one (leave this blank) in order to rotate your private key, in case it has been compromised at some point.