What is public static function create() in plugins for?
create() is a factory method for dependency injection.

Plugins which implement ContainerFactoryPluginInterface are instantiated by create() of the plugin class:

ContainerFactory::createInstance

public function createInstance($plugin_id, array $configuration = []) {
  ...
  // If the plugin provides a factory method, pass the container to it.
  if (is_subclass_of($plugin_class, 'Drupal\Core\Plugin\ContainerFactoryPluginInterface')) {
    return $plugin_class::create(\Drupal::getContainer(), $configuration, $plugin_id, $plugin_definition);
  }

  // Otherwise, create the plugin directly.
  return new $plugin_class($configuration, $plugin_id, $plugin_definition);
}