Almost every site you visit on the web today are dynamic. When we say dynamic, we mean that the server generates the content you see as soon as it gets your request. Dynamic websites have a lot of benefits, mostly the ease of updating, fixing, and deploying a site. However, there are still scenarios where a static web page makes sense:
When a web server generates the content on the page, the process generates a lot of load on the servers. There are ways to reduce the load, but most of them make a small difference to the load on the server. When you have a high-traffic site with very few changes, an HTML site can reduce the server cost significantly. Also, you will not need database servers and other components, just a designer, and a web server.
Another big benefit is the improvement in security that comes with a plain static HTML website. Every single platform and language has security holes and vulnerabilities. The platform needs constant updates and becomes vulnerable as soon as you miss one. However, with a static website, only the web server needs protection.
There are also cost savings from using static websites. You will use less bandwidth, server resources, and fewer servers. Also, you will only need a designer to update the site, rather than a team of engineers maintaining the CMS. A well designed static website will look no different to a visitor, and if anything will probably load faster.