But we also call it the grey text because it appears in grey in your inbox if you're a Gmail user. Well, some of it does; it depends on how long your subject line is. Here's a couple of examples from Halfords and ASOS:
You can probably tell what's annoying me. They have both completely missed an opportunity to display another reason for me to open the email. I'm guessing that the email marketing people at Halfords and ASOS have taken a lot of time coming up with subject lines. They've almost certainly A/B tested different versions to find a winner. They will have had to balance the many reasons why they think I should open this email with the brevity required of the subject line. And that grey text at the top of the email (for Gmail users at least) acts like another subject line. A free chance at a second benefit. And all I see when I glace at them is "cannot see this message" and "unable to view this email".
Why, if you have the opportunity to effectively use two subject lines, would you not take it? Here's another couple of examples from eSpares and Target, this time with the grey text actually used to try to get the recipient to open the mail:
Even if the recipient isn't a Gmail user, it's still the first thing most will see if they open the email, especially if their mail client doesn't automatically download images. This might all seem like small beer; tiny changes that won't really make or break a campaign. But email marketing is all about making as many tiny improvements as you can at each step. A little something to improve the open rate. A slightly better click-through rate by having the call-to-action a bit clearer. Improved conversion rate by making sure the landing pages are as relevant and tailored as they can be. Multiply these small improvements together and you have the difference between a successful email campaign and a mediocre one.