为什么在Symfony2表单中需要字段

Can someone tell me why in form I have a filed that is required:

<input type="checkbox" id="client_invoice" name="client[invoice]" class="invoice-controller" value="1" required="required">

if in Entity I set:

/**
 * @ORM\Column(type="boolean", nullable=true)
 *
 * @var boolean
 */
protected $invoice;

My gues is becouse in form builder I have:

$builder->add('invoice', 'checkbox', array('label' => 'form.client.invoice'));

and then 'required' value is automatically set to true (becouse of the 3rd parameter in add function). Am I right, or is there another reason that this field is required?

Form fields are required by default in Symfony2. You can turn it off for a specified form field by:

$builder->add('invoice', 'checkbox', array(
    'label' => 'form.client.invoice',
    'required' => false,
));