Artic Fuel Heater questions

So I blew a Coolant line that goes to the top of my RHS Fuel Tank. I wnet to replace, and I have never seen coolant going to the fuel tank. I went ahead and replaced the line. While I was confused on what that went there for I called Peterbilt and the service tech said it was probally for a "artic fuel heater" in my tanks.

My question is how exactly does that work?

Other question is, should I turn the valve off in the summer for it, or does it really not matter if I run it year round open?

 

Oh I didn't know it was S.O.P. for the old trucks, heck I thought it was a "bonus". It is a mechanical 425 cat on a 89 Pete 379. Thanks for the help!