One thing doesn't rule out the other. Just because a browser has a built-in adblocker doesn't mean you can't replace it with another one if it's not working well. Every browser should have at least a basic adblocker enabled by default. Anything else is a major security risk. In the context of web browsers ads are the main entry point for malware. Either through exploits delivered via ad banners or by tricking users into downloading something. Many search engines such as Google display fake search results that lead to infected versions of otherwise secure software. Additionally some sites offering downloads have ads disguised as download buttons that lead to something else. A browser manufacturer should try to protect its users from such things.