The C23 edition of Modern C

Important reminder just in the Preface :-)

Takeaway #1: "C and C++ are different: don’t mix them, and don’t mix them up"

Bjarne should have called it ++C.

Because people choose to use pre-increment by default instead of post-increment?

Why is that?

Why use this operator? Like most C and C++ features the main reason tends to be showing off, you learned a thing (in this case that there are four extra operators here) and so you show off by using it even if it doesn't make the software easier to understand.

This is not one of those beginner -> journeyman -> expert cycles where coincidentally the way you wrote it as a beginner is identical to how an expert writes it but for a very different reason. I'd expect experts are very comfortable writing either { x = k; k += 1; } or { k += 1; x = k; } depending on which they meant and don't feel an itch to re-write these as { x = k++; } and { x = ++k; } respectively.

I'm slightly surprised none of the joke languages add equally frivolous operators. a%% to set a to the remainder after dividing a by 10, or b** to set b as two to the power b or some other silliness.

They can be useful when adding things to an array in a loop. A trivial example which removes a character from a null terminated string:

  void remove_char(char *s, char c) {
    size_t i, j;

    for (i = j = 0; s[i] != '\0'; i++)
      if (s[i] != c)
        s[j++] = c;
    s[j] = '\0';
  }

This might be better expressed with a higher order filter function, but C is too low level for things like that.

There are also idioms for stack manipulation using them: "stack[sp++] = pushed" and "popped = stack[--sp]".

C code does a lot of incrementing and decrementing by one, and so having dedicated syntax for it is convenient.