What is the use of function generator?

The Function Generator provides a quick, error-free way to program scripts. You can:
Add Context Sensitive functions that perform operations on a GUI object or get information from the application being tested.
Add Standard and Analog functions that perform non-Context Sensitive tasks such as synchronizing test execution or sending user-defined messages to a report.
Add Customization functions that enable you to modify WinRunner to suit your testing environment.