Pavtube MKV Converter allows you to convert MKV files into other formats. The supported output formats include TS, MP4, FLV, AVI, 3GP, MOV, M4V, MPG, WMV, MP3, WMA, and AAC, among quite a few others. There are conversion profiles for lots of different devices like iPods, iPhones, Blackberry phones, video game consoles, etc.
Basically, you would use this application if you wanted to be able to play your MKV files on devices that don't support that format. There is, however, one feature that is missing from this app that I would totally want in a converter of this type. MKV files often come with multiple audio and subtitle tracks, and this converter doesn't seem to let you choose the tracks that you want to use in your converted files. I have seen other non-HD converters that include this feature.
Despite this missing feature, Pavtube MKV Converter comes with everything else that you might need for an HD conversion: support for lots of formats, a video trimmer and cropper, support for watermarks (image, text and video) and effects.
So, in short, if you don't need subtitle hard-codding or to choose what audio track you want to use, Pavtube MKV Converter is a good choice.