This immersion is on account of the ejection of impurity, so that the vessel will go out from its impurity to the holiness of Israel, similar to a convert who goes out from the domain of idolatry into the domain of Israel. Therefore, even if they are new, such that he bought them from a gentile artisan [who made them], he must [still] immerse them. And it is forbidden to use it without immersion, even for cold [food]. But if a Jew rented or borrowed a vessel from a gentile, it does not require immersion. And this is specifically with culinary vessels, since the verse is dealing with culinary vessels - as it is written (Numbers 31:23), "Anything that is placed on a fire, etc." And what is something that is placed on a fire and and one needs to make it fit (kosher) with a fire? I would say that is a culinary vessel that absorbs [flavor] with fire, and so requires fire for the ejection of what is absorbed.