If you look at the interface in SE24, you'll see it is described as "Tag Interface for BAdIs" That means it is used to identify BADI classes. It's a neat way of grouping classes with a particular attribute. It means you can define a method, (or even a function module) with a parameter of type IF_BADI_INTERFACE, which can be used to process any class which has this interface.
For more about tag interfaces, which is a general object oriented term, see here: http://c2.com/cgi/wiki?TagInterface
There's some SAP specific stuff here: http://help.sap.com/saphelp_nwpi71/helpdata/en/72/581c42f0705033e10000000a155106/content.htm
matt
