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W-9 is the form banks use to collect US citizen client social security numbers in order to satisfy the bank's FATCA reporting requirement.
If you are current on your US filing obligations and want to keep your account open, there is no reason not to return the completed w-9.
If you aren't current on your US filing obligations and want to keep your account open, you'll want to consider regularizing your situation so that you can return the completed w-9 with your tax account in good order.