Those commands effectively make any tcl function be able to operate as an f-expr (in old lisp parlance). Effectively (this is a simplification), an fexpr is a runtime macro, as opposed to the more traditional lispy compiletime macro.

Its what makes tcl feel more like a lisp-for-strings. Much of the truly horrible tcl code out there in the wild is from folks who try to use tcl as just another c-style scripting language.

Treat it more as a lisp, and it some of its inherent elegance shines through.

My $0.02 anyway ...