F.e. when you have more and different ParamConverter per entity you want to name them explicitly.
Then you can use named ParamConverters.

In the documentation this issue is a bit fragmented, so here is the compact version:

Declare:
Set the “converter” attribute in the param_converter service tag.
Read it in the documentation: http://symfony.com/doc/current/bundles/SensioFrameworkExtraBundle/annotations/converters.html#creating-a-converter

You can register a converter by priority, by name (attribute “converter”) or both.

<service id="my_converter" class="MyBundle\Request\ParamConverter\MyConverter">
    <tag name="request.param_converter" priority="-2" converter="my_converter" />
</service>

Use:
In you controller annotation call the ParamConverter with the converter attribute.
The Symfony ParamConverter dokumentation explains it like this:
If an explicit converter choice was made with @ParamConverter(converter=”name”) the converter with the given name is chosen.

use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\ParamConverter;

/**
 * @ParamConverter("post", converter="my_converter")
 */
public function showAction(Post $post)
{
}