The compilerArgs controls the behaviour of the java compiler. These settings are passed on to the compiler, and not used directly by the maven-compiler-plugin. Using Xlint you can control which warnings the java compiler outputs from the compilation process. You can also add e.g. -Werror to abort compilation upon warnings (which in general is a good practice).
The showWarnings configuration on the other hand, is a setting for the maven-compiler-plugin (not passed on to the java compiler). It controls whether the plugin will output the warnings generated by the java compiler. So if you set it to false (I really don't understand why that is the default), you won't see the warnings generated by the java compiler. Even worse, the build will not fail even if you have set -Werror in the compilerArgs.