True, people could, if there was room, move to the left lane. However, if that lane was occupied, as it often was during rush hour, the person had to wait for the right turn vehicle ahead of them. But even if there was an opening in the left lane, the motorist had to look left and back (usually stopped) to check for clearance, then enter the lane. Sometimes this was a quick tight risky maneuver if a vehicle was coming. 
I assume you've had a chance to see what is happening now. First, the bike lane offers a wider radius turn, which allows motorists to not slow so much when turning while they are still in their lane. Second, motorist upon seeing this brief obstruction are taking a few feet of the center lane and going around. They are doing this for buses too. No stopping means less brake wear and better mileage.
Of course there will be very rare limits to this option if the center turn lane is occupied, but I never saw it in 3 hrs of observation at different times (mostly rush-hour).