For short, very fast in development speed.
Spring boot integrated many library I need for server side development. Just add the dependency lib spring-boot-starter* to my pom file, edit the application.yml to config the lib. And I got the basic infrastructure to code.
Actually, Spring boot is a parent for many other spring based product.
For normal web application, I will need spring-starter-web, to use thymeleaf - the greate template engine - add spring-boot-starter-thymeleaf to my maven dependencies. And so on for database, security, message bus (active mq, rabbitmq).
One other great of spring-boot is spring cloud, integrated with netflix cloud. Need to know, even I do not yet deploy all my products to cloud.
And the config file is so important that IDEs support to edit application.yml file, in IDE Intellij IDEA, Spring sts I just press Ctrl + Space to find out the property need to edit. No need to google for 'modify server port spring boot'
Support for spring boot is good on stack overflow. Just add a question, you will sure got answer.
As I write code use other open source software, like Akka or Vertx, I see spring-boot is just a normal lib. No reactive programming, depends on legendary java servlet - that mean a web request will take a java thread for execute. So I can't make any 10k request per second. Even a Hello world!