Super-elevation controls lurch outward. This is important for buffeting. Passengers, sensitive equipment, and perishable goods require stable platforms when being moved, especially at high speeds. Super-elevation is a way of controlling an item's tendency to continue on a tangent, or a straight line, when the platform on which it is being conveyed commences a curved path under it.
If you ever had some learnin' about Newton's Laws of Motion, you would remember that an object at rest stays at rest unless energy is used to accelerate it. Similarly, an object in motion wants to stay in motion unless energy is bled off or converted to induce it to decelerate. We ignore the effects of drag and friction for the moment because they bleed off energy all the time in items in motion.
So, items running hard down a long tangent (rails between curves are called tangents) will want to continue along that notional path, even if the cars they're on have to follow along with the curving track. With abrupt curves, the kind you get in sectional track such as EZ-Track, imagine trying to stay upright while walking down the aisle inside a heavyweight passenger car while the car speeds into an abrupt onset of a sharp curve. Soup would fly, along with their host bowls, passengers slammed against the wall on the outside of the curve. Meat carcasses swaying in a reefer just behind the engine (reefers were at the head end, and on fast freight or passenger consists for quick switching out to another train.) Stacked cases of bananas thrown about, first left, then on the next curve, to the right. Banana smoothie, anyone?
So, for comfort and safety, on the prototype ONLY, rails were both eased and super-elevated. If you wonder how easements work, think of your car as you turn at a city intersection. Are you thrown hard away from the seat because you have to go from tangent to full curve? Or, as you begin to turn your steering wheel, do the front tires also begin to reduce the radius of the curved path? That is what an eased curve does...it reduces the radius during forward motion, thus reducing the lurch. Banking, or super-elevating, contributes to the effect of easing by moving the entire system's centre of gravity toward the pivot point along the radius of the curve at any one point along the curve. Said differently, super-elevation causes a shift toward the inside of the curve, thus helping items to stay where they are due to countering that tendency to get thrown outward.
Easements do help our models, but only on tight curves where the couplers are thrown near their maximum travel sideways along the curve. They do nothing substantial for keeping items upright at the speeds we use. The same can't be said for super-elevation; it can add to the drag on a curve because the physics isn't the same as it is on the prototype. Steep banking will cause the rolling stock on our layouts to ride tight to the low rail, thus causing the flanges to drag and to wear prematurely. More drag, less traction, smaller trains. Do this on a grade, and you're really demanding a lot from our scale locomotives.
So, we ask, why bother to do all the work to both super-elevate and to ease the curves on our layouts? Because it contributes greatly to the illusion that we are looking at a real railroad. For those entering photo contests, it is easy points for that Gee Whiz factor.
