A demonstrative may be used as if the matter were plainly before the addressee, thereby highlighting the hearer's extraordinary failure to grasp it. The rhetorical explanation labels this a notice of extreme obtuseness.
The use is comparable to saying, “You cannot even understand this?” It may carry disparagement, but its analytic point is the contrast between an apparently available fact and the listener's failure to recognize it.
Grammatically the demonstrative identifies a referent. Rhetorically it exposes the mismatch between what is presented and what the addressee understands.