Technical Objections To the Ogg Container Format
E1ven writes "The Ogg container format is being promoted by the Xiph Foundation for use with its Vorbis and Theora codecs. Unfortunately, a number of technical shortcomings in the format render it ill-suited to most, if not all, use cases. This article examines the most severe of these flaws."
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Because, in general, you don't only want a video stream. You want to be able to bundle multiple streams together (usually of different types, eg an audio stream and a video stream). Vorbis, for example, is audio. Theora would be video. Ogg (the container) exists to bundle them together into a single file, so you can have a movie and sound with it.