This. For a period of time in the 1990-1999 range, Tcl/Tk was often the only way to have a cross platform program with a GUI that did not require the language equivalent of #ifdef WINDOWS/#ifdef UNIX everywhere. Which while providing "one code base, two platforms" really meant writing the platform specific code twice.

For most Tcl/Tk usage at the time the same, unchanged, no "ifdef"'s present Tcl/Tk script would run identically on the various environments that it supported.