What you are talking (I assume) is a slave solenoid. Using a Ford style puck relay. Correct?
The reasons are actuality two fold.
First and foremost, it provides battery voltage to the starter mounted solenoid on the GM starter, if you think about it, the power for the dash cones from the battery, which in 90% os the time is at the transom of the boat. So let's say, battery, to terminal, to terminal block, to dash at ignition switch, now downgrade the wire a few sizes and go from the starter, to the terminal block, to the starter that has to pull the Bendix into type flywheel and pull in the contacts to spin the starter motor.
That's over 30' of wire and about 6 terminals. The voltage drop is crazy.
When you run the slave solenoid, all that wire has to do is pull in the easier solenoid, very low current draw. This puts virtually full battery voltage to the GM solenoid, as a plus, if you use the "I" terminal on that solenoid to bump the coil voltage, everything gets a little boost.
I have tested it and have seen voltage drops to the starter to be as much as 6 volts. Meaning your trying to pull in the starter Bendix on half the rated voltage. This is twice the normal current.
This solenoid install will really reduce starter failures.
GT