It keeps it from moving in and out from center. But I different questions. I followed the instructions and used the template to set my rails, The Center support for lengths more than 6 ft is and inch too short? I ended up taken a scrape piece of bottom rail cover and cutting away the side leaving a U channel 3" long, the height it need to by, I left 1" on the top end to screw into the rail bottom, on the bottom side, I lift 5' of the flat part, cutting off the sides, the then crimpted 1" for the bottom, screwed to the decking, and crimpted again, bending the remaining 4' up the other side and screwed to bottom rail
