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$PrePrint is a global variable whose value, if set, is applied to every expression before it is printed.
$PrePrint is applied after Out[
] is assigned, but before the output result is printed.
See the Mathematica book: Section 2.13.1.
See also: $Post, $MessagePrePrint.
Ordinarily Pi and E are left symbolic and are not approximated.
This setting of $PrePrint make numbers that appear in output approximate.
This restores the original behavior.