One converts HDMI audio to Toslink/headphone. Useful, for example, sending Fire TV video to a television HDMI input, and audio to an older receiver without HDMI. $15
The ViewUHD splits HDMI audio out to HDMI-only/Toslink/headphone, as well as HDMI video. I was using this to send HDMI video from a graphics card to a monitor, and the HDMI audio to a receiver. $25