Feature details are small bursts of clarity, activity, and detail. A prominent rose in a bed of flowers, a deep contour of the water, an interesting pattern of dappled light, the subject’s eyes in a portrait. They focus your attention, and in doing so, draw attention away from other areas. This allows you to simplify the other areas without compromising the quality of realism.
This mimics the way we see the world. When we focus on something, the surroundings are muted. You can try it now. Focus on something in front of you. What do the surrounding objects look like? Probably nothing but vague color shapes.
The thing about painting is, you get to decide what is in focus (and what is not). You control where the viewers should look.
I’m going to use Streeton again as an example. He often made clever use of feature details to direct your attention to important areas, allowing him to simplify the surroundings.
Below are photos of his Cremorne Pastoral I took at the New South Wales Art Gallery. Look at the grassy area in particular. Those intricate plants and flowers focus your attention. The rest is nothing more than simple color and brushwork. It’s similar to a magician’s sleight-of-hand. Look at these intricate flowers as I simplify the rest of the grassy land.