In my case, EDA and FPGA stuff. Tools like Mentor Graphic Modelsim and Xilinx Vivado (this one is quite recent by EDA standards) use TCL as both a console interface and scripting language, to the point that configuration files are also written in TCL.

Knowing TCL enables me not only tool automation, but a huge deal of personalization and setup, by allowing me to access data and internal functions of the systems. Modelsim, for example, has its whole GUI made in TCL/TK and I can easily create new GUI elements in TCL to help me debugging.

Also, TCL is a very interesting language by itself. Ok, not modern or cool, but much more interesting than most people think of it. This blog post [1] tries to make this point, and was already discussed in HN [2].

[1] http://antirez.com/articoli/tclmisunderstood.html [2] https://news.ycombinator.com/item?id=7069642