when to use di.xml for my custom module class?

Question

For example i have following two classes.

<?php
namespace Namespace\Modulename\Model;
class Message 
{
    public function getMessage()
    {
        return 'Hello World!';
    }
}
?>

<?php
namespace Namespace\Modulename\Model;

class Example
{
    protected $messageObject;
    public function __construct(Message $messageObject)
    {
    $this->messageObject = $messageObject;
    }

    public function sendHelloAgainMessage()
    {
      return $this->messageObject->getMessage();    
    }
}
?>

I dont need to create below xml in di.xml.because objectmanager take care creation of object automatically from constructor type(means here Message)

 <type name="Namespace\Modulename\Model\Example">
            <arguments>
                <argument name="messageObject" xsi:type="object">Namespace\Modulename\Model\Message</argument>
            </arguments>
    </type>

But my doubt is in which time i have to create above xml?

update

for example please look at <type name="Magento\Catalog\Helper\Product"> di.xml in catalog module.

<argument name="catalogSession" xsi:type="object">Magento\Catalog\Model\Session\Proxy</argument>

if i give directly that class name(Magento\Catalog\Model\Session\Proxy) in constructor instead of in xml?

public function __construct(
        \Magento\Framework\App\Helper\Context $context,
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        \Magento\Catalog\Model\Session\Proxy $catalogSession,

Answer 1

Good practice is to always request for interface in constructor. This way every developer will be able to create totally different implementation of that interface without extending the default implementation.

But when OM encounters constructor that requests some interface it (OM) does't know which exactly implementation of that interface should be provided.

And a way to let it know is to write preference configuration in di.xml:

<config>
    <preference for='MessageInterface" type="Message" />
</config>

Other scenario is when some 3rd party developer wants to substitute default implementation of message in your example:

<config>
    <type name="My\Module\Model\Example">
        <arguments>
            <argument name="messageObject" xsi:type="object">Other\Module\Message</argument>
        </arguments>
    </type>
</config>