I don't understand them and they are never properly explained. There is nothing to get. They're just text, inserted wherever you told the preprocessor to inser

I don't understand them and they are never properly explained. There is nothing to get. They're just text, inserted wherever you told the preprocessor to insert that text, exactly as if you had typed the whole lot in yourself. What you choose to do with this ability is up to you. Many people choose to use it to avoid having to copy the same text over and over again into other files.