Purpose: pointers to tutorials, books, tips, etc. for creating, interacting, etc. Tcl via the C API.
Tcl's original purpose, as detailed in John Ousterhout's seminal Usenix paper (see Ousterhout's original 1990 USENIX paper), was to provide a developer with a standard language that could be embedded in their application. So, in the beginning, the intent was that Tcl would be accessed only via the C API. Over the years, it also became common to extend rather than embed Tcl.