The block-specific download as .png option creates a file that can be dragged from a file directory or from a board post directly into the Blocks Editor, and the Blocks Editor will load the blocks directly into the currently open project, without the need for the user to re-type the blocks.
It's an act of generosity of an author to his readers to provide them with draggable blocks.
See this post for an example of draggable and non-draggable blocks ...