the powerstroke 7.3's used intake heaters in conjunction with the 8 glow plugs.
the glow plugs aid in cold starting. The intake heater is on for a while after it starts. Helps reduce exhaust smoke on a cold start. That is it's only function.
i have one (2003). it starts fine to about 7 below zero. That's as cold as I've started it. Glow it til light goes out, crank, it fires and it goes into fast idle; intake heater (aka toaster) stays on for a while. Glow plugs do too, but usually a few seconds and the toaster stays on for a good while afterwards. I don't know how long. I tested it at roughly 30 seconds and it was still powered.
downside to grid heaters is oil vapors from pcv can foul them, but the cummins guys don't seem to have the issue-or not that I've "heard of" anyway. I think a lot of those guys just forget that they have an engine and are in constant need of fixing the truck more than the engine. I did one today. 36,000 miles on a 21 Ram 2500 6.7L, front end is falling apart. 2 upper ball joints SHOT. bumper mounts falling off. I "think" the entire front crossmember is loose as it clunks terribly in a turn. Ain't looked at it yet. Tie rod is loose. And it needs brakes. They bought it new. It has a 6.7L engine and I assume it's fine, it runs/drives and the engine moves it along ok but the rest of the truck is absolute garbage with so few miles. The cab mounts are plastic. Appears to be PVC. Stock.