An example, if you search for "heroku" then you get an ad for heroku, which, yeah, is what I was searching for, but heroku is also the top result and obviously what I was looking for. Heroku is paying Google to provide Google with the search result that Google should've (and does) already identified as the right result.
I assume the reason these ads exist is because if they didn't competitors would take them, but still, it feels like an odd arrangement very much to Google's benefit where companies pay Google up prevent competitors from taking their traffic.