Flare adds atmosphere of a kind, and also a sense of actuality. Certainly, for strongly manipulated and special-effects imagery, adding flare can be a useful way of helping to convince the viewer that a scene is genuine.
The polygonal flare patterns that are created by internal aperture reflections and refractions from a point source of light (such as the sun) that is either just inside or just outside the picture frame.
From your final 3D rendering, inside your post processing application, you can add your flare. For the most part, you add a flare for a treatment that conveys the intensity of the sun and a general feeling of heat. The software I use is the Knoll Light Factory plug-in for Photoshop.
Keeping the generated flare on a separate layer makes it easy to align with the sun and the rest of your 3D scene.