Data tables can be defined as ones that present relational information (e.g. a bus time table), rather than ones that are used for purely presentational purposes.
While fully sighted users can view all or most of a table at once, and refer back to row and column headings quickly, non-sighted users must access tables in a linear fashion. To aid these users in orienting themselves within a table and in understanding the information being presented, some browsers provide table navigation capabilities. These features depend, however, on table row and column headings being properly identified in the page code - if TD ("table data") is used to code every cell throughout the table, there is no way for the browser to identify heading information and relay it to the user.