The transducer concentrates the transmitted sound into a beam. In theory, the emitted pulse radiates out like a cone, widening as it travels deeper. In reality, beam shapes vary with the transducer type and typically exhibit "side lobe" patterns. The following figures give a graphic representation of the transducer's actual transmit radiation patterns.
Low frequencies have wider beam angles than high frequencies.
For the scope of this discussion, however, the idea of a cone works just fine. The signal is strongest along the centerline of the cone and gradually diminishes as you move away from the center. Wider angles offer a larger view of the bottom, yet sacrifice resolution, since it spreads out the transmitter's power. The narrower cone concentrates the transmitter's power into a smaller viewable area. Cone angles are wider at low frequencies and narrower at high frequencies.
To sum up, a wide cone angle can detect fish around the boat and not just those directly under it while exhibiting less target separation. A narrow cone concentrates the sound output enabling it to better detect small details, such as fish or bottom structure, but only scans a small amount of water at a time.
In reality, beam shapes vary with the transducer type and typically exhibit "side lobe" patterns.