@SpringBootApplication is equivalent of @Configuration, @EnableAutoConfiguration and @ComponentScan. Let's consider why your application works without of any of this three annotations.

Why it works without @Configuration:

When Spring will scan packages, it will find all classes marked by @Configuration and will use them as part of configuration. But in next line you manually passed UserManagementApplication as configuration source:

SpringApplication.run(UserManagementApplication.class, args);

So spring doesn't need to find this class by scan. Therefor it is not necessary to mark it by @Configuration.

Why it works without @ComponentScan:

Class UserManagementApplication has @ImportResource("classpath:spring/application-context.xml") annotation. That means file spring/application-context.xml will be included into configuration. And this file contains next line:

<context:component-scan base-package="com.siva.*" />

So, you don't need use annotation for scan packages, because you already declared it in the xml file.

Why it works without @EnableAutoConfiguration:

This annotation allows to Spring to try guess and configure the components automatically. For example, if you include the following dependency in your build.gradle:

dependencies {
    compile 'org.springframework.boot:spring-boot-starter-data-mongodb'
}

Spring configures all the components required to work with MongoDB automatically. And all you need just specify host and user/pass in the aplication.proprties file.

But you preferred to declare all needed beans manually in the spring/application-context.xml file. So, you simply don't need @EnableAutoConfiguration annotation at all.