Details

Unless $Pre is assigned to be a function which holds its arguments unevaluated, input expressions will be evaluated before $Pre is applied, so the effect of $Pre will be the same as $Post.
$Pre is applied to expressions, while $PreRead is applied to strings which have not yet been parsed into expressions.

Examples

By default, $Pre is not defined, meaning the input is evaluated directly:

Wolfram Language code: $Pre

Save input expressions in held form:

Wolfram Language code:

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

Evaluation works as expected:

Wolfram Language code: N[Pi]

Wolfram Language code: Solve[x ^ 5 - x + 1 == 0, x]

Restore $Pre to its default:

Wolfram Language code: $Pre=.

The variable inputs holds the inputs in unevaluated form:

Wolfram Language code: Flatten[inputs]

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