Why does `$Pre` need to be defined indirectly via some other function?

Question

The documentation for $Pre gives the following example

SetAttributes[saveinputs, HoldAll];
inputs = {};
saveinputs[new_] := (inputs = {inputs, HoldForm[new]}; new);
$Pre = saveinputs;

Evaluating the 3 input cells a then b then Flatten[inputs] gives

{a,b,Flatten[inputs]}

which is the the expected output. For some reason the intermediate step $Pre = saveinputs is necessary, but why? If one tries to define $Pre directly via

SetAttributes[$Pre, HoldAll];
inputs = {};
$Pre[new_] := (inputs = {inputs, HoldForm[new]}; new);

then $Pre does not work and inputs = {}. How is this different?

Answer 1

We can only guess. My guess is that it is only applied when it has OwnValues which your second example does not have.

p.s. you can do it in a one run:

inputs = {};
$Pre = Function[new, inputs = {inputs, HoldForm[new]}; new, HoldAll]