Have you ever found yourself wanting to save your place on a long web page so you could come back to it later and finish reading without having to hunt for the place where you left off? Neither have I, but someone somewhere might have wished for such a capability, at some time. Placemarker was written for that person.
Say you're reading a long article on VanityFair.com - a site that frequently posts excellent articles that are sometimes too long to read in one sitting. What are you supposed to do when you need to interrupt your reading for more than a few minutes? You could just keep the browser tab open, but what if you need to close Safari for some reason? I suppose you could reopen the web page and use the Find command to search for a phrase you remember from where you left off. Sure you could.
Now there's a better way. With Placemarker installed, just select (highlight) a few words at the place where you want to continue reading next time. Then right-click (or Control-click) the selection and choose "Placemark This" in the context menu. Placemarker will create a special "placemark" URL and send it to Instapaper, Read It Later, Pinboard, or Delicious, whichever service you prefer. (You need to have an account with one of these services and be logged in to it, or Placemarker won't work. But you don't need to have the service's web site open at the moment of creating the placemark.)