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Adding Tasks on a Contact to a Campaign

I'm adding tasks to a Contact through the API and have both the realated to and WhatID linked to the associated Campaign(on the task).  This does not seem to associate the contact to the campaign.  I believe I need to pass each contact as into the Campaign Member object to make them offically part of the Campaign.  My question is what is the purpose of the WhatID on the task if it doesn't link directly to the campaign?