What's it's point? - Well it's not just an excercise in vanity, a "Look what I made" statement. It's actually a security measure. The internet is replete with "bad'uns" who are happy to download videos they have taken a liking to from places like Youtube and then re-upload them elsewhere as their own work. This is of course ruined if the true owner of the video has their name or channel indelibly marked into the very fabric of the video.
Unfortunately, the vast majority of nascent video-makers use Windows Movie Maker as their video editor of choice and being a somewhat basic program, has no facility for producing true video watermarks - So does this mean that WMM users will just have to do without Watermarking.