Think of it this way - the engine, airplane, servo rails, servo screw and brass eyelet are all vibrating. The rubber insulates your servo from this vibration. There should be a rubber surface insulating the top, bottom and inside of the hole of the servo mounting lugs. Therefore you need one on the top, one on the bottom. To maximize the insulating qualities of the rubber pads, the screw needs to be snugged down until it just touches the top rubber pad without crushing it.

Ever seen the inside of a doorbell chime? The ding-dong type? You have 2 metal plates which must vibrate when struck by the hammer. In order to vibrate the plates must be isolated from the doorbell chassis. They use larger rubber grommets to separate the vibrating parts from the non vibrating parts. Rubber on the bottom, top and around the inside of the attachment holes.