It’s very annoying when you come across something useful online that you want to access like an ebook or article that states that it is completely free, you just have to provide your email address before you can access it.
When we think of something that is free we tend to think in terms of monetary value. But cost comes in other forms as well, such as our time, and our data. By providing your email address you are giving away information, for free. The individual or company that you are giving this information to may use it to sell you something or to sell the actual data that you just freely gave. There are data brokers, email harvesters, etc, that will pay real money for valid email addresses, especially if it comes with context. For example if you were trying to access a free eBook on investing they now know that your email address belongs to someone that is interested in investing and that is valuable information.
The trick with such sites and asks is not to give your real email address yet still get access to the free content. To do this there are free temporary email services. Just google temporary email to see what I mean. With the click of a button you can create a randomly generated email address with a short time to live in minutes that you can use with the site offering the free content. You can then read the article or download the eBook (obviously scanning it for viruses and malware before opening) and when you are done no one has your real email address. Simples!