Manuel is right. Enclosure = enclosure fan. Platinum = processor fan incl. radiator. You can attach this to the top of the enclosure. The built-in fans are in the front.
The question is very confusing.
Is it necessary to cool the CPU? Absolutely!
Whether the fans in the enclosure are sufficient? Of course not, the CPU must be actively cooled.
And you have already ordered the Corsair water cooling system, which also includes the fans that are screwed onto the radiator.