Can someone please enlighten my lack of knowledge and explain what these do and why and how easy they are to fit ?...
So let me see if I can help somewhat in explanation of what they do.
Basically they stabilize the rear end of the car especially when corning. At one end of the hub is the ball joint, and the stud of the ball joint comes through the hub assembly and then the tie rod fits over the stud as it comes through the other end. As you can see in that video without the tie rod in place you can literally push the hub in and out at an angle.
So think of it this way. Say that both left and right tie rods were worn and needing to be replaced. I go to pass someone, or corner somewhere. When I turn the car the weight shifts, and instead of the rear turning into where I want it to go it slips the other direction. The results could be pretty bad. Actually Maserati did a recall for the rear tie rods of the Quattroporte for this same reason. Literally a person was driving on a highway here in the states and the rear end of the car turned out and spun the person down the highway as if he was steering the car from the rear wheels.
Here check the photos in this link I would repost the photos here but they don't belong to me. The first photo shows the rear wheel turned:
http://www.maseratilife.com/forums/quattroporte/20994-tie-rod-failure-recall.html
I experienced something similar but thankfully it seems the plain spherical bearings wear more uniformed and don't just snap. Well at least I think so because I haven't heard of anything like the Quat yet. While I was driving, I accelerated around a vehicle. When I tried to do so it was almost like the rear of the car let go, like on ice, before it caught, and I gained control. Literally like it turned slightly from the rear of the car. At first I initially thought it was one of the worst performance vehicles I'd ever driven. not realizing of course rear tie rods were a thing for the 4200/GS/3200's. After I checked it out I realized of course the car was unstable because the parts were obviously worn. My wife thought we were going to die to be quite honest. In any case, I hope that somewhat helps you understand what they are designed to do. Basically they stabilize the rear of the car and add performance by helping turn the car into the direction you want to be heading........well when they are performing like they are supposed to.