I see them as visually portraying the reality, what people wore, what the houses were like, bringing to life what we can only see in black and white photos. But like
@Jangles said, the do not show the reality of the class system. When I was a child my family was involved with the restoration of stately house in Ireland. The land was "gifted" to British aristocracy in the 1600's and over the centuries the house was rebuilt and added to and is now very similar to the Downton Abbey House. On the surface it looks like the Big House and the villagers get on, that the villagers were happy working for the family and its basically like being employed in 2022. The reality was very different. Ive heard stories from people who lived in the village when the Family lived in the Big House , stories from the 1950's 1960's. The class system was still very much felt even then.
I look at Downton Abbey and The Crown the same was I look at Gone With The Wind and Roots and other mainstream American historical fiction. They give an insight into the historical events, allow us to learn about events and people which may not be taught in schools but they should be just viewed the same way as a Marvel or Star Wars film.