why is *.ogg always followed up after mp4 or mp3?

Question

in HTML5 course, always the code contains mp3 and ogg or mp4 and ogg.
Should i contain both?

<video controls>
<source src="http://www.sololearn.com/uploads/video.mp4" type="video/mp4">
<source src="http://www.sololearn.com/uploads/video.ogg" type="video/ogg">
Video is not supported by your browser
</video>

Answer 1

the format of .mp4 as well as .mp3 files have licence restrictions. therefore they decided to replace with .ogg and .ogv. you use both so in case one is not supported system picks the other one.