Chromecast Ultra problem


I disabled all trancoding and converting, since my movies are supported. But when i wanted to cast it to my TV it was not able. I had to enable the transcoder and converters (Sorry i didnt test it further). and then i was able to cast… My Chromecast Ultra supports both H264 and AAC which is the codecs i use, why does the transcoders/converter need to be enabled?