Builder Pattern: Can't we simplify the creation of a builder object?

Just getting my head around the Builder pattern and I have a few (noob) questions. Mainly, why can't we just simplify the creation of a builder object?

Instead of this:

public final class Person{

    private final String name;
    private final String description;

    private Person(String name, String description) {
        this.name = name;
        this.description = description;
    }

    public String getName() {
        return name;
    }

    public String getDescription() {
        return description;
    }

    public static class Builder {
        private String name;
        private String description;

        public Builder setName(String name) {
            this.name = name;
            return this;
        }

        public Builder setDescription(String description) {
            this.description = description;
            return this;
        }

        public Person build() {
            return new Person(name, description);
        }

    }

}

//Use it
Person p = new Person.Builder().setName("test").setDescription("description").build();

Why not just use this?

public final class Person {

    private String name;
    private String description;

    public String getName() {
        return name;
    }

    public String getDescription() {
        return description;
    } 

    public Person setName(String name){
       this.name=name;
       return this; 
    }

    public Person setDescription(String description){
       this.description=description;
       return this; 
    }
}

//Use it
Person p = new Person().setName("test").set("description");

I see maybe the first one is used when you want to build an immutable object, but other than that why can't I just use the second one instead? It's far simpler. Please provide examples as much as possible.