No you don't.
It could be handy though depending on the layout of your installation.
In my case, (10 Kw + VEC500 on a 4 T sail boat) I have a manual high current breaker as my main motor switch. It's fairly easy to reach inside the engine room.
A solenoid / relay enables you to place the switch more freely since it only needs thin wires and removes the need for big high current cables to the location of the switch.
