Many authors focus on an era of style because it appeals to them. The society, the politics, relationships, famous people of the day, how they dressed, and what they ate. The entire time period appeals to them, and they absorb it. They may even feel as if they know more about the time period they researched more than the time in which they live.
For example, let’s take a quick look at the Regency Era. Anyone who is familiar with Bridgeton will understand. Afterall, who wouldn’t enjoy researching fashions in this era?

An author may collect multiple old drawings to help them visualize what each of their characters wore. This can be a big help since it makes the characters seem more “alive” to the author as they are writing about them.
The author would also need to research what these ladies would eat while they socialized and discussed how they would bring about a particular martial match for each of their children. They may enjoy tea cakes and finger sandwiches. This may be some of the best research when the author locates this food or makes it themselves to find out what it tastes like. It could be a way to add some vivid details regarding the smell, taste, and feel of their tea.
However, be careful not to get so carried away with the lovely fabrics and tasty teas that you get sidetracked from the real reason you were researching it in the first place. Oh, of course, to write a fascinating and believable book set in the Regency Era. This is the best part of research, to have something wonderful result from all those hours of work obtaining background.
Wishing you Success