Why is a drive letter permitted in front of UNC paths (sometimes)?

A little-known quirk is that the file system accepts and ignores a drive letter in front of a UNC path. For example, if you have a directory called \\server\share\directory, you can say dir P:\\server\share\directory and the directory will be listed to the screen. The leading P: is ignored. Why is that? Rewind to 1984 and the upcoming relea...