Digital inputs are used to connect any non HDMI video gear to the amp for digital surround sound. Won';t handle lossless audio codecs like Dolby TrueHD but works fine with the lossy ones.
Many CD players and streamers have SPDIF digital outputs so you can bypass the internal DAC and use the one in the amp.
Many computers have digital audio outputs or you can add them via internal or USB soundcard.