I know this has been in blender for eons, but I didn’t realize how nice it was until a few weeks ago.
It basically saves you the trouble of re-unwrapping every time you move a pinned vert around. Sure you would just have to press E, but not pressing anything at all is even nicer :)And by the way, if UV unwrapping is a bit strange to you, take a look at a little demo/tutorial/thing I did for a friend:
It also helps relaxing uv maps a lot i noticed. Pin 2 verts than select one and press g, most of the time the uvmap relaxes great without any more hard work
PS this is very usefull for equalizing uv maps now, http://blenderartists.org/forum/showthread.php?294904-AddOn-smooth-selected-uv