YouTube is an extremely powerful website and carries a lot of authority on the internet and especially in Google, since they own YouTube. This means all videos and pages in YouTube can be ranked easier because they are from a powerful website. Uploading your video to YouTube and allowing them to host it, gets your video ranked faster and easier. If you were to host your own video on your website, you would not be able to rank your video as easily as you would if it were on YouTube. Google sees any page or video on YouTube as a reliable and powerful source and will give that video more power.