I did some Tcl work a quarter century agi, but really if I were in the market for an extension glue language today, I'd reach for Lua first.
The biggest reason to use Tcl even to this day is to write Tk GUI programs.
Well... Tcl on its own is good as a configuration language, creating DSLs, or as a scripting/extension interface, as well as a single common cross platform interface to filesystems, network sockets, and more - so I'd say "the biggest reason" depends on what you're working on...