mailManager :send mail at HTML format

Hi there,
In the PhpMail core class, we can found that:

  public function format(array $message) {
    // Convert any HTML to plain-text.
    $message['body'] = MailFormatHelper::htmlToText($message['body']);

But what is for?

I need to send some html mail programaticaly with the mail manager service, but this is prevent some html message, by setting the head like this in the mail hook :
$message['headers']['Content-Type'] = 'text/html; charset=UTF-8; format=flowed; delsp=yes';

like we can found in some tutorial…

what the simplest way to override this ?

Thank !