What is the point of \on-the-fly ?

[David Kastrup]

\on-the-fly gets as first argument a function that it calls on the
second argument as if the first argument was actually a markup command.

Why not make the first argument actually a markup command?

It would appear that we are mostly talking about a closed set here
anyway.  So why

\markup \on-the-fly #(on-page 3) "blabla"

instead of

\markup \on-page #3 "blabla"

?  Where is the point in this particular obfuscation?

-- 
David Kastrup