With the Manual Control, you are in control of the fuel.
With an electrical control like the trucks are wired in with (FSS Solenoid) as long as it works, (and they do for many years) If the Solenoid fails while driving, it will shut you down. Doesn`t happen often, but read where some have had it happen. It is really like a relay to some degree with a winding of wire etc...
With a manual control, takes out one more fail point.
When I bought my Datsun 720 King Cab Diesel back in 81, it had a electric motor, and a small gear box with a wheel, and a rod bolted to it which connected to the IP. Turn the key to start, motor turned the gears to turn the wheel to move the rod to move the lever on the IP to turn the fuel. And the reverse to kill the engine.
Around 4 or so years later one morning it wanted to start but didn`t, wanted to start etc... finally after 6 or so tries, opened the hood and stared at the engine for a bit, and thought maybe that gizmo is the problem. Disconnected the Rod and she started.
A new Gizmo as I remember was $150 or so.
I just popped the clutch or opened the Hood and moved the lever.
Later on rigged up a choke cable to control the fuel.
The EPA fuel was 33 and 39 MPG.
I figured out if I pulled the cable back after getting up to speed on the Hwy, I could cut the fuel and maintain my speed. Any slight up grade had to bump the cable in or she would loose it.
On one trip from Ca. to UT. I got 46 MPG out of one tank while fiddling with the fuel supply.
Wonder if this would work on our IDI`s if we can figure out a manual cable set up.
Goat