This allows previously defined words to be redefined, for instance to include extra code for debugging or other development purposes. One example would be to redefine : (the colon compiler), so that words defined with the new version maintain a count of how many times they are executed, for profiling purposes. How this is achieved is beyond the scope of this introduction.
In fact subroutine nesting in general is very efficient in Forth, which allows definitions in Forth to be very short without significant loss of speed. The examples given in this introduction are not atypical.
In modern stack processors, which implement Forth in hardware, JSR is almost invariably a single cycle operation, and RTS can often be performed concurrently with other operations, so effectively takes zero cycles.