Imagine having the most beautiful website you could dream of. Creating a competitive website is not cheap these days, you have certainly paid a fair amount of money, so it is not a surprise that you want that money back. A lot of websites use advertisements to benefit from.
It is said that an average person in the city is exposed to more than 4000 ads every day. This number is very hard to process, so our brains focus only on those ads, that are relevant for us at the time.