I don't know Tcl much. But when I learned about Tk in Ruby I kind of expanded my knowledge of Tcl since Tk was the GUI toolkit of Tcl that was borrowed by many other scripting languages.

Tcl/Tk had features like distribution of a deployment binary. It could create an .exe file for Windows deployment for example.

Tk included a Domain Specific Language for layout/GUI controls which was useful, without requiring an additional form designer as in other languages.

Others have mentioned Expect above, it was a good reminder, imagine creating a GUI application to control a command-line tool, Expect allowed for that very easily, as it could keep open the output/input of the tool it was adding a GUI to.

Since it included so much by default, the community could easily contribute with some other tools that you could reuse in your solution. Before "RubyGems" even existed, people were already reusing libraries in Tcl/Tk. :-)

The problem of other languages is that they come with more abstractions than people needed. We can fight against "state" in modern programming languages, but it doesn't change the fact that the more complex we make it, the harder it is for people to just get something up and running quickly. That's why systems like Tcl/Tk cannot be retired.