One reason is that it would be paying for a subscription to the company (Google) providing the service (ad-free Youtube). Yeah, it costs money but there's hopefully less work to have an ad-free experience on all your devices.
Using some other third-party app to achieve the same thing might be free but it is attempting to circumvent ads. It's an ethical issue but if you're okay with the circumvention you also should be aware of the caveats (third-party might not support it anymore).