Typically, you build a protector to provide RMS protection for a custom file type. For example, let's say you are building an application for graphic designers and you have a custom file type called .grphc. If you want to apply RMS protection to this file type, you can build a protector (a COM component) to perform this operation.
The protectors we are releasing on the MSDN Code Gallery are written to allow you to protect Office documents, such as .docx and .xls. You do have to write some code to perform encryption and other plumbing, but the code that applies protection to the files is already in place.
Note: If you would like to read about how custom protectors are implemented in SharePoint, visit the documentation.