Have you ever noticed that sometimes pages appear with the wrong modules? For example, your search results may be appearing with all the homepage modules. The Joomla functionality responsible for this is the itemid.
What is the Joomla itemid?
The itemid, along with some other functionality, tells Joomla what modules the requested article or component should load. This usually works great, but when a component or article has no menu item, and therefore no itemid, Joomla assigns a default value of 1. For almost all sites Itemid=1 is the frontpage, so Joomla loads up all the frontpage modules with your component or article awkwardly squished in, if it shows at all.