Why use getters and setters in JavaScript?

I know how getter and setter work in JavaScript. What I don't understand is why we need them when we can get the same result using normal functions? Consider the following code:

var person = {
    firstName: 'Jimmy',
    lastName: 'Smith',
    get fullName() {
        return this.firstName + ' ' + this.lastName;
    }
}

console.log(person.fullName);    // Jimmy Smith
    

We can easily replace getter with a function:

var person = {
    firstName: 'Jimmy',
    lastName: 'Smith',
    fullName: function() {
        return this.firstName + ' ' + this.lastName;
    }
}

console.log(person.fullName());    // Jimmy Smith

I don't see the point of writing getter and setter.

A difference between using a getter or setter and using a standard function is that getters/setters are automatically invoked on assignment. So it looks just like a normal property but behind the scenes you can have extra logic (or checks) to be run just before or after the assignment.

So if you decide to add this kind of extra logic to one of the existing object properties that is already being referenced, you can convert it to getter/setter style without altering the rest of the code that has access to that property.

Edit: Here is an example for an extra logic, this class counts how many times its name is read:

class MyClass {
  constructor() {
    this.refCount = 0;
    this._name = 'the class';
  }

  get name() {
    this.refCount++;
    console.log(`name is read ${this.refCount} times.`);
    return this._name;
  }
}

const myClass = new MyClass();

let maxMessages = 5;
const t = setInterval(() => {
  console.log(`name: ${myClass.name}`);
  
  if (--maxMessages < 1) {
    console.log('done');
    clearInterval(t);
  }
}, 1000);