Buffering costs money! It costs money to the consumer (who conveniently close the video or move to another video) and it costs much more money to the publisher.
Before publishing videos on your servers, its very important that all videos are transcoded keeping the end consumer in mind. There are so many different mobile phones with different screen sizes, which supports different quality of videos. If you don’t support them all, you might lose a large audience who are interested in your content.