In many reports, Users want to see the actual data and to compare this data with the same period for the previous year i.e. for example, if the user enters the current date and wants to see the data from the beginning of the year till today. Also if you want to compare two periods for two different years it is good to keep in mind that working days could be different, so from business side it is important to see the actual working days in the report.
Please check my wiki in the link below:
http://wiki.sdn.sap.com/wiki/display/NWTech/Customer+Exit+Variables+in+SAP+BI