"the permalink icon is used to link back to a specific reply in the forum. When a forum becomes active and a lot of people are replying it can become difficult to direct people to a specific reply. The permalink allows you to copy the link to this specific reply and paste it elsewhere on your network or the internet so that people can find the specific reply.
The reason that clicking on the permalink seemingly does nothing is because it is linking back to the same reply that you are clicking on. So, essentially, you would be refreshing the page.