When a page is rendered, php and mysql are used. Therefore, system needs RAM and CPU.
If many visitors come to a site, system uses lots of RAM and CPU so page is rendered so slowly. In this case, you need a cache system not to render page again and again. Cache system generates a static html file and saves. Other users reach to static html page.
In addition, the site speed is used in Google’s search ranking algorithm so cache plugins that can improve your page load time will also improve your SEO ranking.
In our POPCORN Demo site we are using WP Fastest Cache, WP Smushit, We recommend also Cloudflare with Super Page Cache for Cloudflare Plugin.