Post Type Upload Directory – {post_type}_upload_dir filter

‘apply_filters’ runs the value through all of the other hooks / functions attached to it. Try an experiment:

print_r( $type );
print_r( apply_filters("{$type}_upload_directory", $type) );

While you might, in your case, get the same thing – the apply_filters makes sure that any other plugins that try to influence this upload directory will still work.