> Can you name an important software written in Tcl or Tk?

MacPorts[0] uses Tcl significantly.

EDIT:

The expect[1] command also uses Tcl and has a variant which uses Tk.

0 - https://github.com/macports/macports-ports

1 - https://man.freebsd.org/cgi/man.cgi?query=expect&apropos=0&s...