1.需要session控制的大文件下载，防止因为占用session文件时间太久，导致其他页面的session无法执行

session_write_close() worked as a lifesaver for me when automatically uploading files to a user (forcing a download instead of a link). If files are large, and since session_start() does not allow another page using session_start() to proceed until it‘s done, i was not able to upload more than one file at a time. By using session_write_close() before beginning the file upload, my users can now download as many big files as they like, at the same time. Example:
<?
session_start();
/* Do session stuff here; security; logging; etc. */
session_write_close();
/* NOW write out the requested file. */
header("Content-type: audio/x-mpeg"); /* or whatever type */
header("Content-Disposition: attachment; filename=" . $filename);
header("Content-Length: " . $filesize);
header("Content-Transfer-Encoding: binary\n\n");
header("Pragma: no-cache");
header("Expires: 0");
$file_contents = file_get_contents($filepath);
print($file_contents);
?>

2.保存当前的session变化，防止由于header页面跳转导致修改的session内容没有保存，被丢失

This function is essencial when you change $_SESSION[] variables and then, at some poit in the middle of the script, you send an header("Location: ) function to the browser, because in this case the session variables may not be saved before the browser change to the new page.
To prevent from lossing session data, allways use session_write_close before this header function. session_write_close will force session data to be saved before the browser change to the new page.
Hope this will help you not to loose 1 day wondering why people could not authenticate or make other changes in session vars in your site.