Imagine this scenario: I have a web site where I offer some freeware applications for Windows that are not Open Source (but perhaps are going to be in the future). I'm going to upload a new application that has a plugin architecture, therefore I need to provide the source code for the plugin interfaces that are written in C#. Under which licence should I publish those interfaces? When the interfaces to the plugins are the only piece of source code you are giving away is a licence really necessary?
A license is always needed if the downloader should have more rights than those automatically given by copyright. So without a license, you don't give the interface away, you just allow download of the interface but the downloader can't do much with it legally.
Simple license would be "you are given a non-exclusive, unrestricted license to use the source code of the interface". That's basically giving it away except that all your rights are still intact.