Server side rendering is when React components are rendered into HTML pages on the server after a page request comes from the browser. Static site generation is when components are rendered into HTML pages at build time, so websites can be deployed. Next.js allows you to do both, meaning React components are already rendered into HTML pages by the time they reach the browser.
Improved performance - Components are pre-rendered, meaning less work in the browser.
Optimised SEO - Results in better SEO than traditional single page applications because the response from the server is a fully rendered HTML page.