Unless you include a license that specifies otherwise, nobody else can copy, distribute, or modify your work without being at risk of take-downs, shake-downs, or litigation
short and to the point, it lets people do almost anything they want with your project, like making and distributing closed source versions
lets people do almost anything they want with your project, except distributing closed source versions
Documentation is a love letter that you write to your future self.
Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.