If you need to build a new Apache from source and do not want the result of earlier builds to affect the new one, it is always a good idea to run the make clean command. That will take care of cleaning up any existing binaries, intermediate object files, and so on.
Why are per-directory configuration files useful?
Although per-directory configuration files have an effect on server performance, they can be useful for delegated administration. Because per-directory configuration files are read every time a request is made, there is no need to restart the server when a change is made to the configuration.
You can allow users of your Web site to make configuration changes on their own without granting them administrator privileges. In this way, they can password-protect sections of their home pages, for example.
What do you mean by a validServerNamedirective?
The DNS system is used to associate IP addresses with domain names. The value of ServerName is returned when the server generates a URL. If you are using a certain domain name, you must make sure that it is included in your DNS system and will be available to clients visiting your site.