Sometimes when parts of a painting are too obvious, they lose interest. Even a realistic painting needs to have ambiguity, otherwise it can either be too boring or illustrated. With my students, for example, we always talk about drawing what we see instead of what we think we should see. If you don't see the details in the eye because it is in the shadow, then don't draw it.
When we focus more on the abstract elements of a painting, we focus more on how the shapes, values and colors interact with themselves. In Harold Speed's “On Painting,” he described this idea beautifully. Although he was a realist painter from the early 20th century, he explains how a painting is 100% design, made of shapes, shades, and colors. If all of the elements of art can work together in a piece, you will have a successful painting
One artist that I really admire for using abstract shapes within a realistic setting is Antonio Mancini. Though his paintings are realistic, they are also incredibly abstract. In his painting “Lost in Thought” for example, if you were to only look at the bottom half of the piece you would think it was an abstract painting. And in many ways, ironically, it appears more realistic than any photo.