Nothing to do with the look of a document
All tags together set as a hierachally order form the logical document structure. It has nothing to do with the look of a document or the position of objects on a page. It is the arrangement of meaningful parts and elements, so that information can flow logically from one section to the next.
Backbone for accessible content
The logical document structure is the backbone for machine-readable documents. For this reason it is also the precondition for accessible documents. A PDF document with a logical document structure is called tagged PDF.