YouTube in some regions is slow or blocked. Proxies for YouTube solve it: video loads through an address of a country with full access. Quality and speed depend on the proxy's proximity to the server.
For streaming the proxy must be fast and stable. Buffering and freezes are signs of a slow proxy. Take low-ping addresses.
Watching YouTube through a proxy works in the browser and in the app — the setup is the same: set the proxy, and traffic goes through it. It also works for Smart TV and set-top boxes if they support proxy settings.
And remember: for streaming services take a proxy of the country where the service works. Netflix in one country and Netflix in another — different libraries.