A writing block holds a draft inside the chat as an editable document. When ChatGPT writes something longer, like an email, a memo, or a section of a report, it can put it in a block you can edit directly, select parts of, and ask ChatGPT to change.
That makes iterating faster: "make this paragraph shorter" changes one paragraph, not the whole text. Code gets the same treatment in a code block. When a draft needs to live on and be shared, it can move to a Page.