‘Special Purpose Ledgers’ (FI-SL) are used in reporting. These are basically user defined ledgers, which can be maintained either as GL or subsidiary ones with various account assignment objects (with SAP-dimensions such as cost center, business area, profit center, etc., or customer-defined dimensions such as region, area, etc.).
Once defined, this functionality helps you to report at various levels. Ideally you collect the information, combine it, and create the totals. This is something such as an additional reporting feature, and use of this feature will have no effect on the regular functionalities of SAP.