Do I need to use it? My HP transom assy did not come with it..

As long as you dont put your boat in the water, I don't see it being a problem.

If you leave you boat in overnight anywhere there may be stray current, it's a must. Same for salt water. Cheap insurance.

http://www.sterndrives.com/mercathode.html

Left in the water overnight, If you turn your batteries off I don't think it works.

It's supposed to be wired right to the batt not the shutoff switch.

Wonder why my new HP Transom assy did not come with it. All it has is an anode covering the trim hose manifold on the outside and no wires coming into the bilge.
Boy not another part, I swear I will name this boat "A Few More Parts" Anybody have a spare around they would part with?