By using the different titles (T1, T2, T3), a table of contents is automatically generated when the Story is published/saved as draft. This allows you to navigate more quickly to a section of the Story. It is particularly useful for content-rich Stories.
The optional titles you add to blocks of the Gallery type are not taken up in the table of content. Therefore you are asked to precede this block with a text block with a title if you want to list this gallery in your table of content.