The goal is to detect errors sooner when users will do something like this:

$containerBuilder->addDefinitions(...);

$container = $containerBuilder->build();

$containerBuilder->addDefinitions([
    'foo' => 'bar',
]);

$container->get('foo'); // Exception

Definitions (or anything actually) added after the container was created is ignored, but it sometimes can be not obvious.

By "locking" the builder after the container is created (throwing an exception on all modifications) could help.