Why do some manufacturers include small holes in the lids of their pots & pans??
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I would be guessing if you have any liquid in the pan, such as wine or stock, then the steam hole is to stop the lid jumping up and down when it comes to a boil. It would save you from having to leave the lid off a bit to one side. My parents have set of pans of all sizes like this, and they are their favourite ones.