1) Testing embedded things over comms links.
2) Modelling/simulation, particularly discrete-time simulation.
3) Anything where the ability to generate permutations is important.
3a) At times, generating big tables in other languages like 'C' based on permutations, lists or other data.
4) Operating Excel/Word through OLE.
The thing that Python does not have is 1) packaging ( at least not as well ) and 2) sockets and other comms objects are first-class objects in Tcl.
It's a bit old (it has a 1990s aroma ) for web-based things, although there are web servers in Tcl, most
famously AOL's.