Since my very simple post didn't explain and you weren't happy with it, let me explain. I figured you did enough research to even be remotely be interested in this topic so I thought you'd understand with my previous post. Which still is very important on why I posted it.
The said no-name brand generic springs claim to be linear, but in fact they are not linear which give them the bouncy ride, and bouncy ride means loss of traction and grip. 95% of coilover struts in the current market comes with poorly made springs that are advertised linear springs, but in fact they act like progressive springs due to their material and manufacturing process. Poorly made springs will have irregular spring rates also, so your advertised 8k springs on driver side may act 7k and on passenger side may act like 9k, you can't tune this out even with corner balancing and sway bars! The "advertised/generic" linear spring will changes it's rates very drastically with every millimeter of compression, making them a progressive spring.
Generic springs being progressive isn't necessarily a bad thing either, progressive springs are meant to provide all around comfort and traction over all different types of terrain and weight. Think of motor cycle manufacture's rear springs for a second, they sell the motorcycle and their buyer/rider might weight 120lbs or 280lbs, and they have to make the motorcycle safe to ride for either riders and can't customize all of their motorcycles off the assembly line; solution is progressive spring. The rate of change in spring rate in good progressive springs will be very smooth and gradual upscale if plotted on a graph, and now take our generic coilover springs and put that on a graph, it'll almost look like a cliff on the charts due to it's range of compression.
Now that the obvious things are stated, look at how much the coilover struts compress compared to factory struts, I am willing to bet most factory strut will travel double the distance of what coilover struts will. Let's say factory strut travels 8 inches and coilover struts travel 4 inches, you are asking a lot of traction out of 4 inches of travel.(no pun intended) So you need every little help you can get because you are asking the strut to give you same amount of traction out of 4 inches that you used to get out of 8 inches, the short travel just cannot compare and it does not help if your spring decides to act like a pipe after just 2 inches of compression when you need 4 inches of compression.
Let's not forget that coilover struts are meant for very specific use and YOU have the ability to change the spring rate and damping rate at will for what you are going to be using it for. If you want truly comfortable yet capable coilover setup, use 4k springs front and rear in longest spring you can find to fit.
Swift springs are considered gold standard along with Eibach due to their materials used and manufacturing process ensuring their products act as they are promised.
TL
R Swift and Eibach springs are what they say they are, no-name brand generic springs that come preinstalled on majority of coilovers are not what they say they are and way too stiff for S-chassis.
Also, now that I've told you the benefits of Swift/Eibach, what is your intention with this information? Are you actually going to buy said brands? We still need to talk about actual spring rates and how they should be calculated to be used on a car based on ride height, weight and desired suspension travel.