the left turn signal also tells people incoming traffic that you're turning left, not just the people behind you.
it isn't always clear to the people coming at you that you're in that lane. They're behind the traffic light and can't see the lines as well since your car is hovering over the painted lines indicating which type of lane you're in.
and frankly, it's just good practice to indicate where you're going with your turn signal any time you're going left or right, even if it's obvious. It's just a good habit to have. can't say I practice this all the time, but I do understand the reasons why we should do it.
