If your fishing in a current and fishing from a boat it is easier to direct the lure or bait around obstacles. Say you have a large Boulder in the river. Your up stream from the rock. A longer pole will let you move the lure side to side. The least amount of line that is contact with the water is more desired. Makes for better hook sets. If you let out to much slack chances are your going to snag up. River fishing is all about angles. The more veritical your line is the better off you'll be.

Plus you can slow down your presentation when the current is stronger. The old saying that the shortest distant between 2 points is a straight line. Plays a big part in river fishing. You want constant contact with your bait.