it is there because the monarch wanted it to be there, and it is still enacted as a royal ceremony. See
https://www.parliament.uk/about/liv...ceremonies/overview/prorogation1/prorogation/
The ceremony today begins with an announcement, on behalf of the Queen, read in the House of Lords by the Leader of the House. The announcement states, ‘My Lords, it not being convenient for Her Majesty personally to be present here this day, she has been pleased to cause a Commission under the Great Seal to be prepared for proroguing this present Parliament.’
A Royal Commission consisting of five Peers, all Privy Councillors, appointed by the Queen enter the Chamber, and instruct Black Rod to summon the House of Commons, which he does.