They are not giving "us" anything, Nigel. The treasury decides what to do with it. Including increasing the percentage given to the Royals from 15 to 25 to pay for improvements to Buck House. Now, if there was a clause that stated that income from the Royal estates was to be used to fund, for instance, the NHS, then your statement about the public purse may be true. As it happens though, it is misleading because that income is used ...

"entirely to support The Queen’s work as Head of State.

This means that the money goes towards a number of resources which enable Her Majesty to carry out her official duties. These include: Royal travel for official engagements in the UK and overseas; the maintenance of Royal residencies which are used for formal entertaining and ceremonial events; funding for the work of The Duke of Edinburgh which supports and complements that of The Queen and salaries for employees of the Royal Household who support and administrate the work of Her Majesty as Head of State." (Source https://www.royal.uk/royal-finances-0)