One rook on an open file is pressure; two rooks stacked on it are a battering ram. Doubled rooks defend each other, so the front one can invade squares a lone rook never could, and any piece that dares trade on the file gets recaptured by the second rook, which arrives already invading.
Doubling is also how you WIN a contested file: if your opponent's rook faces yours one-on-one, neither side can safely occupy the entry squares. The side that doubles first breaks the standoff, since a trade now leaves their second rook in command of the whole file.
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One white rook already holds the open d-file. The other is still sitting at home on a1.
Form the battering ram.
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