A configurable plugin can be very powerful. The single plugin will be able to function in different ways depending on the user's preferences. Essentially, your plugin's configuration file will be similar to the bukkit.yml file for your server. It will allow you to change settings for the plugin without modifying the Java code. This means you need not re-build the plugin JAR file every time that you wish to change a small detail. If your plugin is public or used by anyone else, adding a config file may reduce time spent on modifying code in the future. The users of your plugin are able to change the settings that are in the config by themselves, and do not require any additional assistance from you as the developer.
To fully understand why we would want a variable to be configurable, let's look at one of our previous plugins. In MobEnhancer, we set the health of zombies to be 40 instead of 20. Someone else may wish to use your plugin, but they want...