An outline tells you where a shape begins and ends. It doesn't, on its own, tell you much about how that shape actually sits in space — whether it bulges toward you or curves away, whether its surface is smooth or textured, where light is coming from. Value, the range of lightness and darkness across a drawn surface, is what carries most of that information, which is why skilled representational drawing generally relies on value far more heavily than on line alone to convince a viewer's eye that a flat page contains real, three-dimensional form.
Value maps how light actually falls across a form
A three-dimensional object under a light source doesn't present a uniform surface to the eye — the parts facing the light are brighter, the parts turned away or blocked from it are darker, and the transition between those extremes follows the object's actual curvature. Value in drawing is the systematic rendering of that light-to-dark pattern: a sphere, correctly shaded, shows a highlight where light hits it most directly, a gradual transition of mid-tones around its curve, a core shadow where the surface turns fully away from the light, and often a lighter reflected-light area within that shadow where light bounces back from a nearby surface. Reproducing that specific pattern of gradations is what makes a flat, drawn circle read to the eye as a solid, three-dimensional sphere, rather than simply as a circle.
Outline can actually work against the illusion of form
A hard, uniform outline around a shape can work against the illusion of three-dimensional form, precisely because real objects, seen under real light, generally don't have a visible dark line marking their edge — what the eye perceives as an object's boundary is really just the point where one value transitions into a contrasting background value. This is part of why drawing instruction that emphasises value over outline often has students practice rendering a form's edges using only value transitions, letting a lighter shape meet a darker background (or vice versa) without any actual drawn line marking the boundary at all — a technique that, done well, tends to produce a considerably more convincing sense of solid form than a carefully drawn outline filled in afterward.
What we're still unsure about
The general principle that value transitions, more than outline, carry the perceptual information the eye uses to read three-dimensional form is well established in both art instruction and the psychology of visual perception. What's more a matter of individual technique and judgement than settled rule is exactly how a given artist should render value for a specific subject and medium — how much contrast to use, how gradual to make transitions, and how much simplification versus fine detail best serves a given drawing — decisions that depend heavily on the specific subject, lighting, and intended effect, rather than following one universally correct formula for handling value in every drawing.
This sits inside Light, Shadow & Value, one of seven topics in Drawing, one of five domains in Art, one of seventeen subjects the app can quiz you on.