The class 3 barrier keeps the enemy team from getting to whatever you placed it around. There are several reasons that you might wish to keep the other team out. One reason might be to protect the low level skillers from attack while the players that wish to kill take care of them. Another reason could be to keep the other team from taking your teams higher level of clay. You may also wish to build the barrier around a creation kiln so the other team may not use it.