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In CKeditor this is used to determine what block formats should be used for formatting text. Hence no user action is required to spam divs all over the node content, some of them containing non breaking spaces for good measure.
I systematically remove this option from all my wysiwyg module deployments. I propose removing it at the source if others are amenable to it. Requires just changing one line in admin.inc:
'block_formats' => 'p,address,pre,h2,h3,h4,h5,h6,div',