When sending a mass mail to a list of recipients using a normal mail client you don't want to include all the recipients in the To: or Cc: fields, both because it leaks the addresses of all the recipients and because of the risk of reply allpocalypse. That's when a recipient does a reply-all in response to the mail, typically asking to be removed, to all the recpients, and people respond to that and millions of emails later everyone's mailbox is unusable.