I think of Tcl as Lisp for C programmers; by which I mean, Tcl give you the metaprogramming capabilities you get with Lisp in a language that looks more like C, plays well with C, is much more straightforward than your typical shell language, and has a cross-platform GUI. A skilled Tcl programmer can do magic.
I spent ten years, from 2005 to 2015, programming almost entirely in Tcl/Tk and I loved it. Since then I mostly use it for casual scripts rather than for writing apps; but it's still my #1 choice when I need to do some scripting to automate something at the OS level.
I should add...Tk is the easiest GUI toolkit I've ever used (and I've used a bunch of them). It's got all the basic stuff you need, either built-in or readily available. But it comes from the classic desktop GUI world, you have to work at it to make it look nice, and it's a pain to do webpage-like layouts with it.
I wouldn't do an end-user GUI in Tk at this point; but for in-house tools that need a GUI and don't need to be on the web it's hard to beat.
For strictly developer tools, or for those used to older OS desktops or simplistic widgets, it's really hard to beat Tk. It's been around so long that if you've dealt with it before, it might feel dated, but it's also easy to figure out exactly what it does. I got my start using Tkinter with the O'Reilly Programming Python book (the more expanded book from Learning Python), using Python 2.x (probably 2.4 based on the 2008 or so timeframe I bought it). While at that point I had used C#.Net with their GUI builder, as well as Java's AWT and Swing, and Tkinter felt so much more natural when writing a GUI through code.
I spent ten years, from 2005 to 2015, programming almost entirely in Tcl/Tk and I loved it. Since then I mostly use it for casual scripts rather than for writing apps; but it's still my #1 choice when I need to do some scripting to automate something at the OS level.