URI sesgment

[eluser]anis2505[/eluser]
Hi could someone explain the difference between the following

Code:

function afunction($id)
{
      echo $id;
}

And

Code:

function afunction()
{
     $id = $this->uri->segment(3);
      echo $id;
}

should I use $this->uri->segment(3) or it's the same
As I see all people use the uri->segment way

Hope it's clear

regards