Okay, so if you manage your project and all of its dependencies with composer, you won't need the ClassLoader component at all, since composer will generate a configured autoloader that works out of the box. Still, you could use ClassLoader, but why should you?

On the other hand, if for some reason, you cannot (or do not want to) use composer, composer's autoloader won't help you. In my case, I'm modernizing a legacy application and just need certain Symfony components there. Switching this legacy application to composer is not an option right now, so I'm using the ClassLoader component.