Think of Excel
In which you can have multiple pages within a single spreadsheet, each available by clicking that row of tabs at the bottom.
In a "tabbed" browser like Firefox, multiple sites can be opened within a single window, each available by clicking the tabs at the top. It cuts down on screen clutter, even if you don't make as much use of its capabilities as either Robert or Steve G.
BTW, the number of sites that only work in IE is dwindling fast. The only one I go to now is Accuradio.com, and they're working on compatibility.