Most site owners aren’t adept or don’t have the experience to find ads for their sites. Perhaps you don’t have the staff to manage ad campaigns. If you would rather let someone else handle that, joining an ad network is the way to go. You will split the ad revenue based on how many clicks or how many times the ad is views.
MarketingSherpa just published another part in their series about ad networks about what works best. The article is only accessible for a week.
As you probably know, people often don’t click on banner ads, text ads usually perform better. But there is still a place for them and the more targeted they are to the site, the better.
Initial research you need to do.