Framing members repeat on a rhythm, and on-center spacing names that rhythm from the centerline of one member to the centerline of the next. Stud walls run 16 inches on center for typical 2x4 bearing walls or 24 inches for 2x6 walls and non-bearing partitions where the code allows it, following IRC R602 conventions; deck joists lay out at 12, 16, or 24 inches OC. Because sheathing and decking come in fixed sheet and board sizes, the regular rhythm is what makes edges land on framing.
The estimator arithmetic built on OC spacing is a round-up plus a closer: a run of length L needs ceil(L/spacing) + 1 layout members, the extra one closing the far end. A 24-ft wall at 16 in OC takes ceil(288/16) + 1 = 19 layout studs before corners and openings add theirs; a 16-ft deck board over joists 16 in OC crosses ceil(192/16) + 1 = 13 joists, which is exactly the count that sets the screw or clip quantity per row.
Widening the rhythm saves material in proportion — the wall-framing comparison at 24 versus 16 inches on center shows the stud count falling accordingly — but stud size, height, and spacing limits remain a code matter under IRC Table R602.3(5), so the spacing decision is never purely economic.