Apparently they put it in everything!!!.......In my youth I did a bit cooking.....I was doing some cooking in a 'gentlemens club' (it's not what you think....LOL)...and the cook I was taking over from said that I needed to put BOS in to keep the veg 'colourful'....I surmised it was an old wives' tale....as I wasn't taught that at catering college!!!
Back to your question about the Cream of Tartar.... it's used to stabilise and add volume to beaten egg whites. I assume (because I've not made the oopsie rolls yet) that they are made from beaten egg whites???
As said previously, it's found in the baking section near the baking powder, bicarbonate of soda, flours, etc.
Again, I am wondering why I am the size I am...maybe something to do with being so familiar with the baking aisle!!! LoL