The engineer's teleporter is an excellent way for the team to reach the front line. Despite the engineer providing a teleporter, when the engineer need it himself to repair his buildings, the selfish team takes it for themselves because they can't comprehend the idea of teamwork.
The engineers dispenser is vital for the team to survive. Whereas the medic is wasting his time pocketing a soldier and about to die, the engineers trusty dispenser is keeping the team pushing at the front line by dispensing health AND ammunition. The medic is a waste of a class and anyone who plays it should be playing engineer.
The engineers sentry is a defensive item that is used when all the bad soldiers have died and the enemy is pushing, which is, to say, most of the time. The sentry is a gun that can take out the other team, which although the gun seems unbalanced it is almost unfairly balanced out with limited range. However, this does not stop salty soldiers screaming down the mic 'aimbot' because they are jealous of all the kills the engineer is getting.
Whereas all the soldiers are blowing themselves up, the engineers are working together by removing sappers, upgrading buildings and repairing them.