The mobile verison supports "Audio only", but not at Browsers or App on Desktop. While Data-Usage shouldn't be problem on PC - but it actually might - in addition, Twitch uses an non-insignificant amount of CPU-power, which could also benefit from that option (i.e. of Talk Shows, Podcasts, ASRM, Music Streams, ...).
I see no point, to exclude this feature on PCs.