Usual goal with ERP, as accordingly would be stated in business or IT strategy, is to unify the systems, processes and data used in the corporate. Rationale may lay in leveraging development work: costs of development can be split among several units thus improving the business case. Or it may be in leveraging sales: common system enables cross selling, as all sales units can see products and production possibilities across the business. And in similar way the there is greater possibility to centralize and optimize work between units (common purchasing or global supply chain planning), if systems are common and united. 

One major advantage of ERP, at least in some of the market leader products, is in its structuring: the features of product can be divorced from particular choices made by target organization, the configuration. Product can be upgraded on ongoing bases (like virus protection or operating system software updates) as one domain, while still keeping the organizations configuration intact and consistent, as a separate domain.