"A clear benefit" of your choosing? If a republic or monarchy is irrelevant to economic stability then you can't use it as a criticism. We spend a lot of public money on events like today. To many of us they're pompous, patronising, condescending, infantilising, have a fantastic lack of self-awareness and maintain a baffling hierarchy. The "king" is not accountable and we can't get rid of him.
Museums, galleries, old buildings etc are seen as a benefit. So if you're going to eschew the economic question, then what cultural benefit does royalty have for the entire country (and not just your own personal taste)?