why should i use Spring DI instead creating objects by myself? [duplicate]

This question already has an answer here:

• What is dependency injection? 15 answers

i searched a lot about this but i couldn't find any answer that make me see the difference and the benefits of using dependency injection (not just Spring).

So, for example, if i've got this class:

public class Person {

     private ISpeaker speaker = null;

     public Person(ISpeaker speaker) {
          this.speaker = speaker
     }

     public void doWelcome(){
          this.speaker.sayHello();
     }

}

---

public class Josh implements ISpeaker {

     @Override
     public void sayHello(){
          System.out.println("Josh: Hello my friend!");
     }

}

---

So, by my understanding, using injection i would have to inject to the Person class my implementation of ISpeaker, that would be Josh in this case. What i don't understand is why not just do this:

public void myGreetingApp() {
     ISpeaker speaker = new Josh();
     Person person = new Person(speaker);
     person.doWelcome();
}

---

Can anybody tell me the difference of doing this and use dependency injection? and what benefits would it bring me? Thanks !