It's very easy to do something valuable for others. Even a simple gesture of politeness like holding a door for someone may be considered valuable by them. But why does it matter? Let's look into this simple example to get a deeper understanding of how people exchange value.
You are leaving the office feeling good about finishing all the tasks you planned for today and even helping Jane fix a problem in her spreadsheet. You are casually walking towards the exit door and you see someone else approaching it too. You've never seen him before — he is probably a new intern on the marketing team Jane mentioned in the conversation today. So you start going a bit faster and open the door for him. This means that you just spent some energy for this person and made his life a tiny bit easier. He says thank you and goes to the elevator. You won't expect them to return any kind of value to you right now, will you? It would be funny, because they didn't even choose to receive something from you — you just gave it away.
You notice that your shoe came untied, so while he is waiting for an elevator you sit down to tie the shoelace. While you are doing it, the elevator arrives and people come in. The guy you helped notices you and presses the button to make the elevator stay. Now he is giving you value in return by making everyone wait for you. He is taking the social pressure, because everyone can't wait to go home faster and he is delaying them. Then you finally jump into the full elevator and happily go home.
Although you don't know each other — you both just did something valuable for each other. But why did he do something valuable in return? Why did he put the effort into this and decided to wait for you in this uncomfortable situation? He did it because of reciprocity. If you bring value to someone, they are likely to do something in return too, even when they didn't ask for it. It's a simple yet powerful thing. That's why marketers love it. They give you a free ebook, you give them your email — very simple, but in many cases it works. But because this method is so trivial and obvious, it's abused nowadays. I would consider “book for email” a dark pattern. You'll have to come up with something less obvious and truly valuable to get mine.