While one can certainly use their eyes, and make assumptions on bottom contour from surrounding topography, if one uses a depth finder, one will find that often, such assumptions are false.

I fished for many years without a fish finder, and usually caught fish. Now that I've used a finder, I would never go back. I catch more, and better quality fish, by using my brains in conjunction with the artificial intel.

I also used to use live bait, but that's another thread...