[texhax] Ref. Man. Bausum [was tabbing]

[Karl Berry]

    And why would anybody want to program a primitive command this way?

There are quite a few primitives which would be trivially defined as
macros.  For instance, \vfil could have been \def\vfil{\vskip 0pt
plus1fil \relax}.  (Well, actually, \def\vfil{\vskip\z@ plus1fil \relax}
in plain.tex style, but never mind.)

I have always assumed the reason is speed -- that in 1978, the
difference between macros and primitives actually mattered, at least for
things so often executed.  I doubt it would be noticeable these days.

karl