In this example we are moving up a directory and then doing the same action again, the !$ is a short cut to get hold of the ... This is more useful when doing things with longer parameters like directory or file names. For example here we are creating a directory and then moving into that same directory.
Creating a symlink is a common way of ensuring that the directory structure of a deployment will always be the same. For example you might create a symlink so that the release directory of release123/docroot will instead be just current.
Part of the process of putting a new site live can be moving DNS entries around. Prior to doing this it's a really good idea to sort out the Time To Live (TTL) of the DNS record so that when you do change DNS entries you aren't waiting around for a day for the DNS to sort itself out.