From a news report: Ravec had concluded that because Prince Harry was going to be an infrequent visitor to the UK, his security needs would be assessed on a case by case basis.
This is why he is supposed to give notice, so they can assess the risk and sort appropriate protection.
I believe his request to the Home Office was in the hope that instead of this bespoke case by case arrangement, they might decide that he should get automatic protection at all times, regardless of the level of risk. Currently the only people with that level of security are the King and Queen, and the Waleses. This is because of their constitutional importance.
However - if the case by case threat assessment for a particular visit determined that Harry needed 24/7 armed security, he would still get it now. The risk is assessed each time he comes and appropriate protection provided. If on the other hand he is judged to need the same arrangement as the King, he wouldn’t have to give notice and the risk wouldn’t be assessed each time, it would be automatic. So he could turn up at a moment’s notice and have 24/7 full armed security which wasn’t actually necessary. That’s what he wants.