I use the chrome browser. Maybe your mileage is different, depending on what browser you use?
Whenever I open a tab and that tab is on some particular webpage, there is usually some sort of icon associated with the webpage, which displays on the tab. (If it is a new tab that is not linked to any page, then the tab just says "New Tab".) The icon is determined by the website. For example, if I were to go into Facebook, then the tab would have the facebook icon on it. These icons allow you to look at all of your tabs simultaneously, and easily find the webpage that you are looking for (presuming that you know and recognize the icon), without having to open every single tab.
Back when the site was still ATP, all of the pages had a friendly leaf icon on them. Right when the site became the National Gardening Association site, the icon changed to one associated with the organization. Unfortunately, it was a poor icon - it was in low contrast pastel colors (which did not show up well against the grey background of the tab), and the figure/icon was pretty much indecipherable and unrecognizable.
Since I typically have a lot of tabs open (right now, over 60 (I stopped counting)), I want easily recognizable icons on the tabs. And since the more tabs that are up, the smaller the tabs are, a crisp and clean and high contrast (to the background) icon is necessary for easy readability.
