The larger the transducer cone angle, the wider the sonar beam going to the bottom. If you fish shallow water mostly, a wider cone will allow you to see more fish. But in deeper water the wide cone may show you fish that are too far from your bait to be enticed by it.
Also, figuring out the width of the cone at a given depth is one of the few examples where I know trigonometry has helped me outside the realm of pure science

I use a ducer with a 20 degree cone angle for water mostly 20 feet and shallower. I would not want one much narrower for shallow water.