While you're correct that the {brackets} around the variables aren't necessary in this case, they're still perfectly legal and necessary in cases where ambiguity exists.

$var = "${one}two$three";     # "1two3"
$var = $one . "two$three";    # "1two3"
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Certainly both of these methods achieve the same result, but there are instances when putting two variables together in a double-quoted string is more legible or needed for other reasons. In these cases, surrounding the variable name in brackets is necessary.