Crush SO2 DetailsWhy adjust SO2 at crush?
Grapes are covered in wild yeast and bacteria, most of which would render grape juice unpalatable if left to do their thing. Thankfully most of these strains don't like SO2, so you can do your wine a favor and and some SO2 to the crushed grapes to inhibit these strains.
The wild yeast that you want to survive (assuming you're a daring soul who does not inoculate) can survive in the presence of some SO2 and not be bothered.
SO2 can also protect juice from some enzymatic browning (due to polyphenol oxidase for you enzyme geeks, you know who you are). Note that SO2 has no effect against the browning enzyme Laccase (produced by the Botrytis mold.)