Apis: The APIs are used to store the user data in the database or handling authentication.
Markup: Site markup can prebuilt during the build time by using a static site generator.
Each time you click on a new page or blog post in the WordPress site we make a network request to the server and we need to wait until the server builds that page and sends back to the client it often sometimes server fail to build a webpage during the heavy load.
The hosting cost is also very cheap for the Jamstack sites compare to the other tech stacks.(Lamp or mean etc).
The site you are currently browsing is also build with jamstack by using the Gatsby static site generator.
they are many more sites.
I have a complete tutorial of how to build a jamstack blog by using gatsbyjs and react.