Go back through the last four tracks and count the systems one change order touches: the RFI log, the PM tool, the estimate, the accounting system's budget and commitments, the pay app spreadsheet, the WIP workbook, the owner report. In many companies, each of those is a separate copy, typed by a different person.
Every copy is a chance to drift. A number keyed twice will eventually be keyed differently, and month-end becomes a reconciliation project: which version of the estimate is right, which COs made it into the WIP, why the pay app does not tie to the ledger.
A single source of truth means each fact is recorded once, where it happens, and every report reads that same record. For example, a change event can be traced through pricing and approval to related project records. Each handoff still needs review, including the effect on the company books and WIP.
In os.construction, Financials follows a project's budgets, commitments, changes, pay apps and cash. Company Accounting covers the ledger, payables, receivables, payroll and controls. These workflows share project context, but a project pay app is not itself proof of a posted receivable or cash received; review and reconcile the records.