Using the @Override annotation is useful for two reasons :

Noncompliant Code Example

class ParentClass {
  public boolean doSomething(){...}
}
class FirstChildClass extends ParentClass {
  public boolean doSomething(){...}  // Noncompliant
}

Compliant Solution

class ParentClass {
  public boolean doSomething(){...}
}
class FirstChildClass extends ParentClass {
  @Override
  public boolean doSomething(){...}  // Compliant
}

Exceptions

This rule is relaxed when overriding a method from the Object class like toString(), hashcode(), ...