Can some please explain why we need a Head of State

Leaving aside the argument of whether we should have a monarchy or not* - the main objection to moving away seems to be that we'd then have an elected president, and OMG President Blair would be the worst thing ever.

I am confused by this. It seems to me that at the moment The Queen doesn't actually _do_ anything from a political point of view. Why, then, does she need to be replaced _at all_. What is the function that she carries out that actually needs doing? If she could be replaced by a robot that said "I hereby approve of this law that was voted in by the houses of parliament", "Well done for winning, you are now the government" and "I totally understand that you're fed up being the government, I accept your resignations" then do we actually need a head of state at all?

Can someone explain to me the need for one**?

*Seriously. Anyone turning the threads into "But they bring in lots of money from tourists" or "It's morally wrong to have two classes of citizens" will find their threads frozen/deleted. I'm looking for the answer to a question here, not a republican/monarchist flamewar.
**And by need I don't mean "Wouldn't it be lovely to have someone the whole population gets to vote on." or "They get to travel the world raising the UK's profile" - that's what pop stars are for.

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