Who wouldn't want that?

It shows on mobile devices just below the subject line. On some desktop clients like Gmail, it can show to the right of the subject line, depending on display settings.

Used carefully, it can mean the difference between an email open and not.

It must be padded with special characters to stop unwanted text appearing beside your pre-header. 

Other platforms have a similar approach.

PS Most mobile devices will show 32 letters or less, so keep the visible text under 32 if you can.