She serves as an important 'symbolic' head of state.
Yes, and we are all her "subjects", and pay for the upkeep of her properties and for state events in her honour. Most unsatisfactory.
It's a simple deal: She gets to keep her vast country-ruling powers, on condition she never uses them.
Don't forget all the free money from taxpayers, even though she is the richest woman in the country. Not sure about her but her son is a tax dodger too.
Whenever I hear the 'free money from taxpayers' argument, I feel compelled to point to the following: The true cost of the Royal family explained. Regardless of whether the stuff the article at the end of that link says about how much money they bring in for the UK is true or not, I'd still be quite happy paying the 65p per year to have a monarchy.