The Tcl/Tk license allows binary-only redistribution, which is one key difference compared to the GPL, making it amenable to incorporation into closed-source software.
The Tcl/Tk license, in contrast to the GPL, does not require that derived works be released under the the original license: You can start with a piece of code from the Tcl/Tk/tcllib/ sources that does something similar to what you want, modify it to do exactly what you want, use that in a larger project, and still be free to choose the license under which you release this larger project.
Kbk writes in comp.lang.tcl the following summary of the Tcl license:
In even simpler language: Paragraph 1. It's ours. Paragraph 2. You can use it, modify it, give it away, or sell it. But only if you tell people about these terms. Paragraph 3. Don't sue us if it doesn't work. Paragraph 4. We don't have to fix it, either. Paragraph 5. Even if the Government uses it, we can use it ourselves, give it away, or sell it.