It Isn't So Much the depth as it is the tides.
When you have the world's second highest tides (Cook Inlet AK)you have to have jigs that will still reach the bottom where many of our fish species live.
When the fish get big and 100 pound fish are not uncommon but the tide runs hard and you're fishing deeper than 150 ft you need a jig that will get down there and get the job done.
Of course we often times try to fish when the tides are mild in areas where the water isnt as deep but work schedules and weather often times dictate where and when we can fish.
When you're fishing for bottom-dwelling fish if you can't reach the bottom with your presentation, well, you're just not going to catch them.