If luck gave you a ISP offering a good DNS service there is still a matter of privacy and escaping surveillance.

Laws ordering ISP to keep logs of their customer activity are becoming increasingly common around the world. By using a different DNS server your ISP DNS server has no logs to show for your DNS activity.

It is good privacy practice to split the data about your internet activities across different providers to make it more difficult to track you. For example if you use google services for web search, then you should not use google for your email (ideally your should host your own email).

But the sad state of reality is that ISP DNS are often poor, whether it is hijacking DNS for profit or blocking domains following judge's orders or local laws.