The short answer
Buy off the shelf when the business benefits from a proven standard: finance, tax, payroll, localization, inventory controls, or a familiar industry workflow.
Build or configure a custom layer when your advantage depends on a process the standard product forces into workarounds, duplicate entry, spreadsheets, or permanent consulting tickets.
Do not confuse custom with coding everything from zero. A durable platform plus generated extensions and a maintained external authority is often the strongest 2026 choice.