I think kcott is saying that you might, in some conditions, wish to retain the non-empty false value in $x rather than discard it in favour of $y. eg. if 0 were a valid input for $x and you had:

$x = 0;
$y = 10;
$number = $x || $y;
print "$number\n";
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You would see 10 rather than the valid-but-false 0. In such cases $number = $x // $y; might be more appropriate.