Squinting allows you to view your subject with less detail thus increases your sensitivity to value changes and color variations. It simplifies shapes, values and color by reducing distraction details.
By including a few squints, you can concentrate on large, basic, overall shapes. Squinting reduces colors to values and simplifies them into highlights and shadow masses because it eliminates much of the middle values.
This simplification of your shapes and values will help to unify your art and will compel you to focus on your composition rather than on the details.
TIP: If you wear glasses, take them off to blur your focus.