why use signed apk in android code example

Example: how to generate signed apk of android project

android {
    ...
    defaultConfig { ... }
    signingConfigs {
        release {
            storeFile file("my.keystore")
            storePassword "password"
            keyAlias "MyReleaseKey"
            keyPassword "password"
        }
    }
    buildTypes {
        release {
            ...
            signingConfig signingConfigs.release
        }
    }
}