You are helping the advertising websites and they are returning the favour. They are able to pay you because their advertisers pay them every time they send you a promotion or ad. Each advertising websites has a contract with MNC's advertisers, that their product should be seen by large number of viewers all over the world. The Email your receive contains advertisement about a product or service or idea. The multinational companies want their advertisements to be viewed by large number of peoples all over the world. It creates many things like popularity, Brand Loyalty, Better image and so on.
DO I HAVE TO BUY SOMETHING TO GET PAID :
No. The websites pay you to view the ads of their advertisers who are advertising on the internet due to its wider reach. Just like on TV you view ads but you hardly buy that product. But the more popular its product become and it aerates a market of its own.
