Giving things away for "free" is a common marketing strategy to generate leads. A lead is a qualified contact of a potential customer. This things could be of a broad range of cheap assets, e.g. ebooks, apps or even games.
I have seen a marketer buying the copyright of a ten year old textbook for a cheap price, polishing it, giving it a new shiny cover and a new title containing up to date buzzwords, and then giving it away for a "free" subscription via e-mail, and on top asking the customer kindly to come up for the shipping. I wondered why i couldn't find this book on amazon or somewhere else, but then i understood that it was re-released on the purpose of being exclusive for subscription. The cost for printing a small number of copies (500 copies) is quite low ($1-3 per book). This is a decent price for a qualified lead.
This strategy has two benefits. First, you get a qualified lead with contact info and even with a correct address. Second, some countries have laws against e-mail spam, but this laws often have loopholes. In cases that your company has a business relationship with someone, you are allowed to get in contact and advertis your other products via e-mail, on an opt-out basis. Giving "free" assets to a costumer establishes this kind of business relationship. Almost always this "free" assets are of a minor quality, for me not sufficient enough to give away my datas.
Freemium isn't free.
Just wanted to point out that this question is decidedly not hypothetical. Many websites use third-party services to administer their mailing lists.
Now, this makes sense from a developer's perspective since we don't all want to be rewriting mailing list tools, but I'd wager many users aren't aware that their email address is being passed on to a third party. These are the sort of tradeoffs that we're now learning to address explicitly, as opposed to sweeping them under the rug in the name of "marketing". As much as it's a pain for me as a developer, I welcome it wholeheartedly as a user.
It just went into effect - has this been measured already?
Don't give your email if you don't want spam... that include this website.
The author has spent hours creating a useful guide that he is giving away for free in exchange for an email address. He likely hopes to tap into this audience later for a paid product.
Reaching this audience via email is cheap and easy. He can email them as many times as he wants, even if he doesn't want to sell them anything.
But because of GDPR, he can't depend on email so much. He has to rely on other tactics to reach his audience again.
And what are those tactics?
Paid advertising, of course.
No one stands to benefit more from GDPR than Google and Facebook. Everyone here cheering for GDPR needs to think how it placed all the advertising power away from mediums you can control (email) and over to ad companies.
As a consumer, you can unsubscribe and block emails easily. As a creator, you can set up a mail server easily.
You can't do that with ads, at least without relying on outside tech