The title attribute can be used with all elements except for base, basefont, head, html, meta, param, script, and title, but it isn’t required for any. Maybe that’s why it’s less clear when to use it.
Use this to provide additional information that is not essential. Most visual browsers display title text as a tool tip when the element is hovered over, however it is up to the browser manufacturer to decide how the title text is rendered. Some will display the text in the status bar instead. Early versions of Safari did this, for instance.