Wine Making Sulfites, such as Potassium Metabisulfite, has many uses. Sulfites are normally used to stabilize wines by killing off spoilage bacteria and indigenous yeasts that could be present in fruits used and in the atmosphere. The amount of SO2 used is enough to inhibit the growth of unwanted organisms, but will not interfere with a cultured wine yeast since they are more resilient to sulfites than indigenous organisms.
Essentially, sulfites enable your fermentation to start as cleanly as possible. Sulfites also help with the prevention of enzymatic browning of wines which in turn helps maintain the complexities of wine - as well as assisting later with storage of wine after bottling. Sulfites, when used at proper levels, will protect wine by preventing organisms from thriving, and also scavenging oxygen while in storage.