The appearance of a third domain of life-forms, Eukarya, matters a lot to us because all large organisms, including ourselves, are built from eukaryotic cells. These were the first cells that could use oxygen systematically, exploiting its fierce chemical energy in a process known as respiration, which is what we do when we breathe. Respiration is the reverse of photosynthesis and is really a way of releasing solar energy that has been captured and stored within cells through photosynthesis.
