The is used to mark interruptions in thought and brief summaries of content.

dash

Explanation:

Summary of content, which can also be called overview of content. A dash can be used to mark interruptions in thought and brief summaries of content to the sentence that are closely related to the content  but not grammatically part of that content. Example of such can be a restatement or an illustration  etc.

obama was told in early july 2004 that he was chosen to deliver the address, and he largely wrote the speech himself, with later edits from the kerry presidential campaign.

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