As has been pointed out, you can certainly catch fish without a depth finder.  But say you want to fish 20 to 30 feet of water as you troll.  If you are too shallow, your lure will hit bottom and you know it is less than 20 feet.  But if the water drops off to 60 feet, I want to know that I am now in water much deeper than I intended to fish.  Plus sometimes it is cool to know the water is 200 feet deep.

Westwood