Grapes are covered in wild yeast and bacteria. Thankfully most of these strains don't like SO2, so you can do your wine a favor and add some SO2 to the crushed grapes to inhibit these strains.
The wild yeast that you want to survive can survive in the presence of some SO2 and not be bothered.
SO2 can also protect juice from some enzymatic browning. Note that SO2 has no effect against the browning enzyme Laccase (produced by the Botrytis mold.)
