The great thing about Spring Boot is no need for an external servlet container. All that is needed reside inside a single runnable JAR file. In a very few steps, development of a new application can be started without installation or configuration of any additional software.
Yet, sometimes you might want to deploy your application to some server as a regular WAR file. For instance, you convert an existing application and want to keep your continuous delivery pipe untouched or a particular container is enforced by a company’s policy. The reason for building a WAR file may vary across teams, but for development purpose a simple executable JAR file with an embedded server might be preferable.