The Accounting Schema defines the default accounts to be used for system generated transactions (e.g. accounting for AR Invoices). These accounts are used as default values for Business Partners, Products, Warehouse, Project, Bank, Taxes and Cash Book. These can all be overwritten at the individual level but having these defaults ensure that every transaction is properly accounted.
The objective is to generate all accounting transaction entries. Consequently, all necessary accounts are predefined. For example, if you sell a product, the revenue account defined for that product is used. This allows you to define the accounts a single time and then you do not have to be concerned with the accounting consequences when entering documents.