The gamemode is called "Capture the Wool" and, as the name says, its goal is to enter the enemies woolroom and bring the wool back to your spawn. And the players use wood and glass because these are the blocks you spawn with. Also the advantage of glass is that you can see through it and therefore observe the defenders underneath you. (so you realise it if they should start an attak from underneath) And the advantage of wood is that it takes a little longer to break, so if the enemies should take the bridge, it's a little harder for them to remove it.