With plain Tcl/Tk, the widgets do not automatically come with scrollbars. If you want to use scrollbars with the text, canvas or listbox widgets, you will need to create scrollbars separately and attach them to the widgets. This can be a lot of hassle because you would almost always need scrollbars for these widgets. Sometimes you will wonder why you need to write the same boring code again and again just to get the scrollbars to working.
The Tix scrolled widgets are here to make your life easier. With a single command such as tixScrolledListBox or tixScrolledText, you can create a listbox or text widget that comes automatically with scrollbars attached.
Another advantage of the Tix scrolled widgets is that you can specify their scrolling policy so that the scrollbars appear only when they are needed. This feature is especially useful if you are displaying a lot of widgets and running out of screen real estate.