If you achieve United MileagePlus Premier Status, you earn more United MileagePlus miles as a Premier member when you fly United.
Members earn 5 miles per $1 on fare. Premier members earn more. Silver members earn 7 miles per $1 spent on flights, Gold members earn 8, Platinum members 9 and 1K members 11 miles per $1. Premier members also get more miles when they fly with partners. See details here.