LuckyDuck, it's because you want to minimize the amount of line above the water. The dive curves for dipsys and TTs are for the amount of line that's actually in the water. For example, let's say you're running a size #1 dipsy on zero base plate setting and let out 31 feet of line because we want to reach a target level of 20 feet. Well if your line counter is reading 31 feet but you're rod is in the vertical position and there's 12 feet of "wasted" line before its even touching the water, then you really only have 19 feet of line in the water and are therefore running your bait a lot shallower than you intended. Veteran trollers out there, please correct me if I'm wrong...