Fuel for bacteria. Similar to vodka dosing. Also, depending on whether Kalk is being used, vinegar allows the kalk to supersaturate the ATO water. Only so much Kalk is soluble in RODI. But if you add vinegar, more kalk becomes useful.
BINGO! In the example above by luisgo he was looking for a carbon source to lower N & P in his tank and he determined Vinegar to be the safer of the bunch. It just happens that using it in your ATO reservoir allows you to super-saturate the water with Kalk, I'm probably going to try this in the near future.