Because it helps to prevent you from getting boxed. I read an article where Seb Coe said the stay on that line. So many runners (Keely) hug the rail, get boxed and are stuck.
Grant Fisher is a case in point. Yes he was in the 5 and 10 but he always hugs the rail and has no room to move when he has to.
He should have medaled in both of his races.