Magento 2: what is the $data array constructor parameter?

The $data[] can be used to hold values passed from di.xml's arguments replacement mechanism, when \Magento\Catalog\Block\Product\ListProduct intantiated.

In your case this could be populated by

<type name="\Magento\Catalog\Block\Product\ListProduct">
         <arguments>
             <argument name="data" xsi:type="array">
                 <item name="0" xsi:type="string">anystring</item>
                 <item name="1" xsi:type="string">xyz</item>
                <item name="2" xsi:type="string">abs</item>
            </argument>
         </arguments>
     </type>

You can even pass object and other formats to an array using above di mechanism. Further you can use those arguments while extending functionality. We can say, it is a kind of va_list in c.

This can be used when you don't want to modify the number of arguments in a constructor and want to inject other classes in your extended functionalities.

The $data can be used to populate data on your object since the constructor of \Magento\Framework\DataObject is this

public function __construct(array $data = [])
{
    $this->_data = $data;
}

or similar for Magento\Framework\Api\AbstractSimpleObject

/**
 * Initialize internal storage
 *
 * @param array $data
 */
public function __construct(array $data = [])
{
    $this->_data = $data;
}

which a lot of classes in Magento extend from.

A common use is in conjunction with a factory. For example in Magento\Sales\Model\Order\CustomerManagement we have:

$this->addressFactory->create(['data' => $addressData]);

which essentially populates the $data array at creation.

Having to keep the $data = [] at the end of the list of parameters is normal php behaviour since you are assigning a default value - the empty array.