Your film gets potential larger audiences than if you just host it on your own website. When you spend plenty of time creating your film, you want it to be seen and have as much exposure as possible. Youtube has search features, allows people to give feedback and rate your video which can be useful to you. Plus, youtube handles the streaming and bandwith for you, saving money from using your own websites bandwith and cost of streaming features you may need.

Similar sites more dedicated to the serious film maker such as www.pawky.com also gives you a community of film makers to be a part of. Of course, having your own website with your video downloads is good too.