It's a common practice to place copyright notices, various legal disclaimers and sometimes even full license agreements in each source file of an open-source project. Is this really necessary for a (1) open-source project and (2) closed-source project? What are you trying to achieve or prevent by putting these notices in source files?

I understand it's a legal question and I doubt we can get a fully competent answer here at programmers.SO (it's for programmers, isn't it?) What would also be interesting to hear is, when you put legal stuff in your source files, is it because "everyone does it" or you got legal advice? What was the reasoning?