Left-turn lanes have a very specific purpose, which is to remove traffic from the main travel lanes while cars wait to turn left. In some cases, they have to stop at light-controlled intersections and either wait for a green arrow or wait until traffic is clear to turn. In other cases, the left-hand turn lane may just split up what would otherwise be a two-lane road, and turns can be made at any point -- such as a person turning into their own driveway.
Drivers and cyclists often have a bit of a heated relationship. They are expected to share the same roads, but they often act more like they are competing for that space. It can lead to some dangerous situations when no one wants to act in a civil manner.