It is used in daily conversations, but as a standalone sentence or a standalone clause. For example: Joe: "Who says I have to do it that way?" Mike: "Everybody says so!" Joe: "I don't have to do it that way just because everybody says so!" It is used when a statement is made--"so" refers to that statement. It means that the majority of people have said or at least believes it to be true. In the above example, it is synonymous with, "Everybody says that you have to do it that way."

Perhaps 'daily' is an exaggeration? I don't think I would even use it yearly. I would consider it as something that less capable people would use to justify their opinion, when they are incapable of justifying it any other way...