We all know how irritating it can be to see advertisements as pop-ups on a website. Why do they exist? Let us take a look at some online services. What do they do and are they doing it for free? Shortly, yes, they are offering something for free. Their service is fully or partly for free.
They actually need to get money on some way. Offering services for free is not easier than offering them for money. We can try to do some kind of analysis here. Let us take some examples, just to be able to understand this subject. You are probably familiar with e.g. Youtube or some online newspaper.
We all know that their services are available for free. There might be some kind of limitations like age or the place where we are, but the services are for free. Maybe the online newspapers offer less than Youtube but the idea behind their business is the same.

The online newspapers give us some parts of the content, but for the full version they require membership and payment. Some of them offer full content but with disturbing advertisements whole time. Those can even cover the whole screen during a specific time. Youtube offers music or movies but with many disturbing interruptions. Their advertisements can drive us crazy. However, we can skip them if we become a premium user.
No matter what, the online services need to survive and therefore they need to earn money through disturbing advertisements or paying customers. Other ways, they would not be able to give us what we want, regardless if it is a reliable content or not.