Spreadsheets are excellent for calculations and poor at coordination. Once several people update the same information, the spreadsheet becomes a source of disagreement rather than a source of truth.
Watch for repeated data entry between tools, reports that take days to assemble, and decisions delayed until someone reconciles the numbers. These are coordination failures, not effort failures.
An ERP helps when the same customer, order, stock item or invoice needs to be visible to more than one team. If your teams are re-typing what another team already recorded, consolidation will pay back quickly.