由 Thomas Bertels （開發者） 發表於 Sept. 5, 2009 · 永久網址

Because some people (including me) don't want to have to click precisely on a tab to close it. I find it easier to click on the top right corner of the screen to close the tab without even looking at it (like I do for windows).
Additionally, users who don't want to bother with difference between tabs and windows just click on the top right corner and what they expect happens : the page is closed.