The SPL is a user defined ledger which can be built to support reporting requirements which cannot normally be met through the usual SAP modules.
You can pull information from many SAP modules such as FI, CO, MM, SD and build user defined fields whose contents are based on calculations from other SAP standard information.
Typical uses include:-
Reporting using an alternative fiscal year variant other than the one assigned to the entity.
Reporting in a different currency (maybe to meet a new head office reporting currency).
Meeting USGaap and local reporting requirements.