There is no coherent streaming protocol available for desktop browers yet. HLS and RTP are only supported by mobile devices or stand alone video players like VLC – but not by the majority of current desktop browsers.
Thus, using a Flash player was required so far, to enable desktop browsers to play streams coming from a streaming server. The used protocol is RTMP.
The RTMP protocol has been developed by Adobe and is a proprietary protocol. It is the quasi standard for streaming servers meanwhile and requires flash.
The big advantage of HLS is that no Flash player is required any more and streamed videos can be played by the VIMP HTML5 player directly.
HLS support works with almost all current desktop browsers and should be used by default, if you use VIMP together with a streaming server like e.g. Wowza.