I was casting to the player then noticed the player index was 0 by default.
What is the use of it?
It is just a default value, any count starts from zero 
You can change it if it will be needed
What is the use of changing it??
If I change to 5 what will happen??
If you change it to 5, you won’t find the player’s controller because by default it is equal to 0.
It canbe used to have several , for example for local multiplayer