Contour trolling will allow you to present your bait right in front of the walleyes nose. In cold front conditions this is essential. What you're trying to do is stay on a particular depth, or contour, where it looks like the walleyes are holding.
Walleyes are a structure oriented fish, most of the time.
You might find large schools on some Great Lakes that don't relate to specific structure, but by an large they seek out structure. These walleyes will be tight to the bottom, laying in the holes between rock and cuts in the bottom. They may be feeding, or waiting in ambush to find an easy meal that comes their way. When fishing structure, you have to be able to stay tight to the structure or your lure presentation will not be in the strike zone of the fish. Move just a boat length away and you will be out of luck.