Probably when it's a website, not a webapp.

Client-side app libs/frameworks are ideal when you are building something that's interaction-heavy and sending lots of small bits of data back and forth to the server. Or, if it's querying large data sets and reflecting rapid filtering, that kind of thing. The tradeoffs of frameworks are usually acceptable - heavier initial load, javascript required, no SEO concerns as it's all sitting behind authentication anyway.

If you're building a traditional website that approach is less valuable, as you're sending relatively large chunks of information down one way and with little need to change it. Combine that with needing SEO for your public/searchable content and you find yourself building universal/isomorphic apps - which is a really long way around to server-side rendered content.

There is no one-size-fits-all solution to the web. Start with the requirements of the thing you are building - what do the users need, what problem are you solving for them. Work backwards from that and choose technology that supports solving the problem.

You should't use react if common sense tells you it's not necessary. Use the right tool for the job.

That's the most emotional question I have ever read here :D Congrats