It's not only for the final proof-reading, though that helps, too. It's getting through it all at once at as close to a readerly speed as possible. This way you can catch inconsistencies. You can also catch where you thought things needed to be explained, or foreshadowed, and forgot that you had already explained or foreshadowed them so that you were doing it over and over and over again -- your reader will notice even if you didn't because the insertions were weeks apart.
Of course, these chunks may just need excision, which doesn't prevent this being the final pass, but may need rewriting to fit in smoothly. In which case it probably isn't the final pass because you have to slow down to rewrite. . . what fun.