The headline (or the subject line) is the first thing your readers see. Grab them with a great one, and they’ll keep reading. Bore them and they’ll move on.
The purpose of the headline? Capture attention and get your readers to read the first sentence, which has to be interesting enough to get them to read the next one. And so on.
Read these posts for guidance on writing great headlines (without tearing your hair out).
Proven Headline Formulas and Why They Work
Get Great Headlines Without Writing
How to Write Headlines Your Readers Can’t Resist