MKV files are very popular. For example, the very popular, great (and free!!) DVD / Blue Ray / to some extent, HD DVD ripper, MakeMKV (I can only recommend it!) uses it as the only target format because it's probably the only format to keep all the features (menus etc.) of a DVD. This so-called “container” (that's the MKV) is far more advanced than anything else out there. For example, it can store not only goodies like chapter info (including name) and menus, but even individual files (for example, a text file explaining what is on the video etc.) and any kind of subtitles.