Two way left turn lanes are designed to reduce traffic congestion and rear-end collisions when turning left mid-block while saving the space otherwise required for a dedicated left-turn lane in either direction.
Two way left turn lanes are often used as acceleration lanes by drivers entering the highway. This is not legal in British Columbia. If you are driving in a two way left turn lane, the proper way to leave it is by turning left. The arrows painted on the pavement and the overhead signs require this.