Why we need an immutable object?

I want to explain it with 2 examples.

First one:
class A{

private StringBuffer name;

public void setName(StringBuffer Name){
name=Name;
}

public StringBuffer getName(){
return name;
}

}

class B{

A a=new A();
StringBuffer temp=new StringBuffer("Anai");
a.setName(temp);
System.out.println(a.getName());
temp.append('a');
System.out.println(a.getName());
}

output:
Anai
Anaia

It is completely against "encapsulation" concept.It is mainly because of mutable class(StringBuffer).

Second one:
see the immutable class which i mentioned it on my previous post.Suppose you want to maintain table of past data which never going to be changed then this kind of implementation will be helpful.

Main Advantage of Immutable is ThreadSafe