While you wouldn't want to leave "console.log" in your code in production, you can use them in function components just like a classes' render methods.
A better example (from the video "React Today and Tomorrow and 90% Cleaner React With Hooks" from October[1]) is something along the lines of updating document.title = `${some} Page` or possibly calling an API.
The console.log was only there to illustrate that the code in useEffect's callback is only being run on the client-side (see the codesandbox I linked to).
I think it's clear from the parent I was responding to that they have better use-cases in mind :)
also btw: https://reactjs.org/docs/hooks-faq.html#do-i-need-to-rewrite...
and I still have no idea how I only do stuff on the client in a SSR environment (stuff that i did in componentDidMount)