Because some of the videos against YouTube's Terms Of Service are too good to lose!
We believe if you can stream a video, you should be able to save it.
If you are like us, you think it makes more sense for you to have the option to watch videos either online streaming or download them to your computer for more reliable offline playback.
If you download a video you can enjoy it without worrying about buffering, streaming issues, or internet problems. Of course that's all well and good but just being able to download videos isn't enough.