ERP systems deliver a group of business processes like General Ledger, Human Resources, Inventory, MRP, etc. But these processes are one size fits all. For example, a business rarely makes money on the General Ledger but every business uses it.
Most businesses can use whatever model is available that conforms to good accounting practices.
For ERP software the justification is that the sum of the whole is greater than its parts. But this can also mean that a part may not be as good in the ERP suite as it is in the legacy system. Some will lose so that all can win. Of course, if the business makes its money from a specialized business process, it is not a good candidate for an ERP module. The business will then follow the same model as every other ERPed business and lose it's competitive advantage.