agent1953,
Yes you could have used the stock transducer that came with your 798ci HD Si unit but you would have needed some sort of adapter to mount it. The made for the trolling motor transducer comes with the hardware that you should need to mount it to the trolling motor plus it has some additional shielding built into it to help shield the Si sonar from trolling motor interference.
When the trolling motor is pointed straight ahead the unit/transducer is scanning the Si sonar 90 degrees to the right and 90 degrees to the left. If the trolling motor is turned 90 degrees to the left; than the unit/transducer is scanning to the rear of the boat on the left Si sonar and to the front of the boat with the right Si sonar.