When dealing with a server-side rendered application (through frameworks like Gatsby or Next, or any sort of SSR setup), it can be useful to know whether you're rendering on the server or the client.
For example: If you don't have a dynamic backend (eg. a Gatsby app), you won't know whether the user is authenticated or not. In that first pre-render, you should render a blank spot. When the cient renders, you can fill it in, either with the user's avatar or a "Sign in" link.
Immediately after the very first render, we set some state, which immediately triggers a re-render. This has a performance cost, but it's necessary.
Some developers try to get around this "double-render" by using a ref instead of state, to check whether they're on the server in the first render:
This is a bad idea; it can lead to really wonky bugs.
For more information, check out The Perils of Rehydration. It explains why this is a bad idea in much greater depth.