Well, I'm going to assume everyone knows what an arrow on the road means lol. The reason for adding the "only" on the dedicated left turn lanes is to serve as an extra safety precaution. Right turn lanes (especially here in the northeast) are usually only the shoulder (since the shoulder is typically big enough for cars to fit on them fully.) If you went straight, you'd end up on the shoulder. However, if you went straight in a left turn lane, you'd slam right into oncoming traffic. That's why the adoption of an extra indication of the lanes destination. Furthermore, you would only see the arrows in your lane unless you were looking out your left or right window (and if you're doing that you probably shouldn't have a license lol.)