Protected left turns (a signal indication with red arrow, yellow arrow, and green arrow) are less efficient than protected/permitted left turns (a signal indication with red circle, yellow circle, green circle, yellow arrow, and green arrow) because motorists can choose to turn left on a green circle if there is a gap in traffic at a protected/permitted signal indication. Basically, more left turning vehicles can get through the intersection with protected/permitted phasing versus protected phasing.
So why do we have protected left turn phasing? Some traffic engineers think they are safer, mainly because they don't trust motorists to judge an appropriate gap in traffic. There is some validity to this theory at intersections with high speeds or poor sight lines (safety report). The argument doesn't hold at flat intersections with a 30 mph speed limit.
As I previously wrote about (here), Mn/DOT is working on a hybrid with a flashing left arrow that could give us the best of both worlds.