Re-opens the page from which a tab was initially opened.Backtab either jumps back to the page from which you opened the current tab, or re-opens it if you have already closed it. This works no matter how long ago you closed the page (Chrome itself only remembers the last 10 closed tabs incoherently).
If you do a lot of online research, reading through documentation, browsing discussion groups or just Wikipedia, you most likely end up with a lot of open tabs. At some point you end up in a tab asking yourself: