The next example associates the same header (TH) and data (TD) cells as before, but this time uses the "scope" attribute rather than "headers". "Scope" must have one of the following values: "row", "col", "rowgroup", or "colgroup." Scope specifies the set of data cells to be associated with the current header cell. This method is particularly useful for simple tables. It should be noted that the spoken rendering of this table would be identical to that of the previous example. A choice between the "headers" and "scope" attributes is dependent on the complexity of the table. It does not affect the output so long as the relationships between header and data cells are made clear in the markup.

So I think the scope causes the TH to be read each time... otherwise you would have to remember and guess what each section means when it is read, quite hard with many rows, so scope causes it to be read each time so it is clear what each TD holds for info.

The scope attribute in the table header will connect any cell (td) within the column with the header (th) for that column. Each time a screen reader encounters a data cell within that column it will read the table header for that column followed by the data in the cell.

I've never seen the id used like that until now. I've always used scope to associate data with table headers. And I'm wondering why the example uses both id and the scope, it seems redundant from what I've just been reading in the WCAG from the link Chris gave.

By having an ID it also allows that header to be directly spoken to for filling it with database info or allow links/screen readers to jump directly to it.