Sometimes you see a Web page whose creator was lazy and did not wrapped displayed URL into <A href="..."></A> tags. In short, you see something like
http://www.thinknostic.com/ instead of http://www.thinknostic.com/ (btw, if you have installed Linkify, you will see both as links). As result, you can see the link on the page, but you cannot really click on it. In order to follow the link, you must select the link text, copy and paste it into address bar and press Enter. With Linkify installed, you will always see all textual links as real links and can just click on URL as if the page would contain the proper <A> tags.
Linkification shows it's presence by small icon with letter L in status bar - tooltip on this icon shows how many links were converted. Right click on icon will allow to set options, and specify for example which protocols should be recognized.