Things placed close together look related. Spacing shows what belongs with what.
Objects near each other are perceived as a group. Spacing communicates relationship.
A sign-up form mixes account and delivery fields, all evenly spaced, with no boxes, lines or headings. Drag the fields into order and open the space between them until the eye sees two clear groups.
~8px within a group, ~32px between groups. Aim for 3–4 clusters of 3–5 fields.
Put a price too far from its plan name and it drifts toward the neighbouring plan. Is that $29 for “Pro” or for “Team”? Tighten the spacing so each plan's name, price, feature list, and button hug each other, and the grouping is obvious without a single divider line. The whitespace does the work borders used to.
Proximity can fight other signals. A legal disclaimer tucked right under a “Buy” button gets read as part of the button, so sometimes you deliberately want separation. And in dense data tables, tight grouping can merge rows; alternating background bands often group better than spacing alone.
A signup form lists 14 fields with equal spacing. Completion is 31%.