Surf the web for guides on how to make money from your site, and almost every single one will start will talk about mailing lists. It will be either most important thing or one of the first you will be asked to do. None of these guides are wrong. Any website that does not maintain a mailing list of their visitors is missing out on a lot of potential income. Here is why:
You might not always rely on advertising
Advertising is often the most common method of monetizing your website traffic. You get paid by clicks if it is an ad network like adsense. Alternatively you could get a flat monthly fee for coverage on your website. But someday you might want to sell your own product. A huge potential market for that product would be your past visitors. There are visitors who like your site and content but need reminders and motivation to return. That is where a mailing list will help.
You might need help with your products
Even when building your own products for sale, a mailing list can be very useful for shaping that product. You can start with a poll or contest that will give you invaluable insight into what your visitors want and are will to pay for.
There are a lot more reasons to start your own mailing list. The two points above are just a starting point that can lead to more income and traffic.