When you ask people why they believe it or do it they have no justified reason for it--- only "because that is the way it is done".
It should always be kept separately, even in a tank that doesn't have a single plant in it. You know, that idiotic idea about keeping your mechanical media in until it disintegrates? There is no logic behind it--- it is a mantra that has been repeated so many times that, instead of analyzing it, it is just done. When you ask people why they believe it or do it they have no justified reason for it--- only "because that is the way it is done".