I don't mind admitting that I earn a few bob from YouTube advertising placed by Google on my videos. It's a nice little earner on top of my salary as a Lecturer. But I am a complete miniscule novice compared to the earning power of others on YouTube. According to Statista, PewDiePie earned $15 million in the six months to June 2016, even the tenth highest earner pulled in $5 million - impressive.
Source: you will find more statistics at Statista.
A quick look at the list above made me realize that I had no idea who any of the top 10 were - I should get out more often! PewDiePie's channel looks interesting - he has 50,597,217 subscribers and 13,951,262,373 views. Clearly, a few people are making a lot of money, but I'm guessing that the vast majority of YouTubers earn very little from their channels. Very few videos "go viral" and make lots of money. Nevertheless, good content will get noticed and it may be possible to add a few hundred euro a month to your salary. Educational videos will not get you near the top ten or make you a living, but there is still money to be made.