First things first. Harry and Meghan are not *buying* Frogmore Cottage. They're approved by HM, The Queen and the Crown Estate as as being suitable tenants for the property. Frogmore Cottage is a Grade II listed house in the grounds of Frogmore House built in 1801 and held in trust for the British nation as part of their history. Yes, the Queen loves being at Windsor Castle but the fact is that its not *her* castle. She's the current resident is all as is her prerogative. Charles, as King may never spend time there at all during his reign. Its personal preference. It may surprise you just how little property the British Royal Family actually "owns". In a nutshell its Balmoral and Sandringham. Back in the time of the abdication of Edward VIII, George VI had to wangle how to get them back for Britain Its a story worth delving into. Those are their "piles".
Frogmore Cottage has been chosen as the "home" for Harry and Meghan much like Bagshot Park is the home of the Wessex family and Royal Lodge is the home of Andrew. All in the same area. They're given the use of the properties and they do pay rent and pay for the upkeep but the property will always "belong" to the Crown Estate.
Making Frogmore Cottage a residence again is doing something. Its preserving it for history. Harry and Meghan are just going to be temporary residents for the time being.