this is same as test parameters in QC, since most of the times we write tests as a template that can get lots of parameters values that will trigger the same test expected results (e.g. equivalent classes, automation) we need to have the optio for a 'place holder' (like dates, values) that will be turned to valid data in the test set (AS AN INSTANCE).
the benefits are
1. reuse of tests
2. ability to write tests as early as possible
3. it is unecessary to wait for reference data before writing a test
4. work with additional file repository (such as in GUI testing).