How to understand Gradle dependencies block?

Question

I am new to Gradle and have some confuse when learning Gradle.Say, I have a denpendency config:

dependencies {
    classpath 'com.android.tools.build:gradle:2.3.3'
}

I realize that classpath is a configuration for dependency and the closure after dependencies is delegate to DependencyHandler object, and I referred the api of DependencyHandler found that it has a Dependency add(String configurationName, Object dependencyNotation); method， but why there is no add here? why can we add the dependency just like configurationName "group:name:version:classifier@extension" other than

dependencies {
    //classpath 'com.android.tools.build:gradle:2.3.3'

    add ('classpath', 'com.android.tools.build:gradle:2.3.3')
}

Anyone some explain please.. Thanks