Dry fly fishing on broad spring creeks is one reason. The added length helps with mending the line and helps a bit if you need to reach out a bit farther. I also use a 9' 3wt often when fishing pocket water even at close range. When high stick dry fly fishing the longer rod helps with keeping all of the fly line and all or most of the leader off of the water. Some of the high mountain stream that I fish may be narrow and shallow but I might need to kneel 15-20 feet from the water to avoid spooking the fish so a long rod helps hold the flyline off the ground while just the leader is in the water. Angling for panfish on ponds is another situation where I use a longer 3wt rid.