Back in the 1960s, shows like Doctor Who and The Avengers were broadcast once and then usually destroyed or taped over. So there are lots of missing episodes from those early shows. But every once in a while, one of the strays turns up sometimes because a fan recorded it at home (a more arduous process in those pre-VCR days), sometimes because a copy was shipped to an overseas affiliate and never returned.
As of today, the number of missing Doctor Who episodes has dropped from 108 to 106. The two recovered episodes one from Galaxy 4 and one from The Underwater Menace arent exactly at the top of fans most wanted list (probably the last episode of The Tenth Planet, or any episode from Marco Polo, The Massacre, or Power of the Daleks, would fit that bill better), but hey, well take what we can get.