Spring Boot enables the developer's to quickly develop an Application by using the Spring Framework by taking care of default configurations and allowing Java developers to focus on rapid prototyping. Tomcat is the default embedded server for Spring Boot. So you you don't need to setup tomcat when developing Application using Spring Boot.
Also, when you run your normal Spring Application on Eclipse, Intellij or any Other IDE, you setup Tomcat or any web server in the IDE itself. IDE also manages all the things for you like deploying war file on Server and running the Application. But if you want to deploy your application on some hosting server, you would have to make some necessary changes to your Project.
This post shows you how to Install Spring Boot Application on Web Server like Apache Tomcat, Oracle WebLogic Server, Jetty and or any other server.
Our main method delegates to Spring Boot's MainApplication class by calling run. MainApplication bootstraps our application, starting Spring, which, in turn, starts the auto-configured Tomcat web server embedded in Spring Boot. We need to pass MainApplication.class as an argument to the run method to tell MainApplication which is the primary Spring component. The args array is also passed through to expose any command-line arguments.