Well, with the regular wing loading the optimum target for any general type of design varies with the size. A wingloading that is considered feather light on a 6 foot span model would be considered as being in the lead sled category if that same design were scaled down to 3 foot.

The whole cubic loading calculation is one way to take the sizing into account. If you scale the model so that the cubic loading remains constant then as you cut the 6 foot model in size to a 3 foot version and run the calculation for cubic you'll find that the weight needed to achieve the same loading is automatically lowered to a suitable amount.

So for some folks that like it it allows for a quick check of what a scaled up or down design should weigh if they are not familiar with that sort of model in the new size.

For those that fly or look up data on similar models at the new size it's easy enough to find data on what represents a suitable wing loading for that size and style of model.

Niether method is "better" than the other. Just different.