A configurable plugin can be very powerful. A single plugin will be able to function in different ways, depending on user 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 that you need not rebuild the plugin JAR file every time you wish to change a small detail. If your plugin is public or used by someone else, adding a config file may reduce the time spent on modifying code in the future. The users of your plugin can change the settings that are in the config file by themselves and do not require any additional assistance from you as a developer.
To fully understand why we would want a variable to be configurable, let's look at one of the plugins that we previously talked about. In MobEnhancer, we set the health of zombies to 40 instead of 20. Someone else may wish to use your plugin, but...