The aluminum cladding acts as a protective layer to the wooden frame. It will be protected from weather and rotting.
If your garage is at the front of your house, the garage door is likely the first thing people notice when they pull up outside (after all, they do account for approximately 30–40% of a house’s facade). If you don’t clad the door frame and you fall behind on maintenance, the first thing people are going to see is flaky paint, and that’s really not attractive. By cladding the door frame in aluminum, you’ll increase your curb appeal.