When I warm my diesels in the winter it has several benefits in my mind;
1) the pistons have a chance to warm up which reduces piston to wall clearances, resulting in much less noise, piston slap.
2) the oil has been circulating and warming up the entire engine which is resulting in more uniform oil flow with less pressure and the pressure relief is not opening as much so more oil is circulating.
3) the hydraulic pump on my open center equipment has been circulating and warming up, my hydraulic controls respond smoother and easier.
4) depending on the tractor or truck I may run the transmission, I'll put one transmission in neutral the others in gear and clutch out or shuttle shift engaged which further circulates and warms the system.
5) after a warmup period the tractor or truck just feels better, runs smoother, responds better, and the controls feel and function better.
6) the biggest one I feel that my equipment will last longer work better and be nicer to me if I am to it.
I also use block and manifold and reservoir heaters as well as synthetic lubricants and blended and treated fuels.
I would much rather stay in the cab working then be outside working on the equipment.
Also I'm sure I missed several reasons to.