The syntax helps with evaluating the expression inside it.
$arr = @(1,2,3)
$msg1 = "$arr.length"
echo $msg1 # prints 1 2 3.length - .length was treated as part of the string
$msg2 = "$($arr.length)"
echo $msg2 # prints 3
You can read more at http://ss64.com/ps/syntax-operators.html