What is the purpose of 'mark as read'?

This option allows you to mark the messages you do not want to read and see displayed on the page as read. To mark a message(s) as read, check it (them) using the check box(es) situated to the left of it (them) and click on mark selected messages as read.

If you are interested in none of the messages a certain section contains, instead of using the check boxes, click on mark the rest of this section as read. This will mark the messages as read and, provided you have selected the only new mode (see the top of the page), they will no longer be displayed. If you select all, the System will keep displaying all the messages regardless of whether you have read them (marked them as read) or not until they expire.

There is also the option of mark the message as unread placed at the end of every message. Clicking on the option causes the message to appear and act as unread in the future.