Paying websites are many.There are around 900 Paying websites from which you can earn upto $5000. These websites pay you to view the advertisements . Just like on T.V you view ads. It creates market of its own.
You are helping the advertising websites and they are returning the favor. They are able to pay you because their advertisers pay them every time they send you a promotion or an ad.
Each advertising website has a contract with the MNC advertisers that their products should be seen by a large number of viewers say about few million viewers.
The Companies across the world want their Ads viewed by large number of people all over the world. It creates many things like Popularity, Brand Loyalty, Better Image and so on.

They are paying you because, only then you will click and read their Advertisements continuously.