Do not have a picture? Many different types of screens. Some screens are traveling and the angle allows the material to stay on the screen as the screen lifts up and away for cleaning.
For bar screen, the angle causes the trapped material to be pushed to the top through the action of the flowing water. A worker can then rake off the trapped material.
The primary reason is to make the screen somewhat self cleaning. This works where the screen is in a channel so the water approaches with significant velocity in one direction. Slanted screens are much less effective and less common where water is being sucked from a motionless reservoir.
