One field of use for #tcl that I think people might find surprising is that it mixes well with #nix , at least to me.
Tcl in #nixos has few dependencies, so you can use it for miscellaneous scripts without feeling bad about it.
When doing Nix stuff you can easily get into a few layers of quoting, something Tcl's syntax handles better than the shell code people instinctively reach for.
Tcl excels at orchestrating and composing system commands, again a typical Nix concern.
Finally, even though the programming model could be said to be the inverse of functional programming, on a superficial level I think Nix code and Tcl have a shared aesthetic, they just look good together. π