On the fly customisation. Obviously these lines are very useful to the C-compiler, but no use in say an HTML file. Using a HTML preprocessor allows you to easily maintain a consistent look and feel for your web site. All style, colours, fonts, formatting etc. can be written as macros which not only simplifies the web design process, but also means you can get a whole new look and feel for your entire site by simply changing one file. Makefile - The Makefile used to generate this page.
Most software allows the user to enter parameters to configure the software's behaviour.
These parameters often have a default value. The user can specify a value for this option when starting the program. If no value is specified a default is used. The default value will be documented somewhere, normally in a man page and possibly in any other documentation which comes with the program. If the developer changes the default value in the code, they must also remember to change the value wherever it appears in the documentation.