The pre-header text is shown immediately after the subject line of messages and it’s displayed in the preview snippet in email inboxes.
Without a pre-header sentence the first thing that is displayed in emails is often a “view online” link and salutation. This doesn't entice readers to open and read your email.
Email in an iPhone inbox without pre-header text:
The same email with a pre-header text gives your contacts a reason to open your email