It can be used to give emphasis. You can easily emphasize a اِسْم by just adding the corresponding personal pronoun in its normal form after the اِسْم which you want to focus on.
This form is called ضَمِير الْفَصْل – as it separates the subject from the predicate and provides some space in between.
Let us have a look at some examples:
It is Khalid who sits there.
خالِد هُوَ الْجالِسُ هُناكَ.
The engineers were the ones responsible for the success of the project.
كانَ الْمُهَنْدِسُونَ هُم الْمَسْؤُولِينَ عَنْ نَجاحِ الْمَشْرُوعِ.
I have done my duty.
قُمْتُ أَنا بالْواجِبِ.
He wrote the lesson himself.
كَتَبَ هُوَ الدَّرْسَ.