In ancient times, when youtube was born, we didn’t have a VIDEO tag. We used to have Flash. To play video on the website, we had to convert it to weird Flash video format, leave mp4 for safety and hope that everything would work fine. Later on, when HTML5 became an industry standard, our lives became much easier. In theory, VIDEO tag solves all our issues. When the browser can handle webm (Chrome family) we play webm.