Bafflement Chronicles: Part 1

A continuing series of things that baffle and bemuse me.


Footless Tights. 

Now, I sort of understand plain old leggings, and own a couple of pair (used for layering under clothes during winter or exercising in the home). But tights and even footless nylons? i.e. "Items one wears with skirts, but are not substantial enough to wear on their own."? These, I do not grasp the use of.
Most of them, especially the more popular "Spanx" ones (and the knock offs) are usually of the following description. They generally start at the waist with a fairly firm control top, and then become sheer about mid-thigh, then there's another non-sheer 'cuff' around mid-calf where they end. Nude colors, or they come in black and patterned black. That being the case:

1. So you can't wear them with a skirt, unless they are longer than the mid calf. Unless you wear them with boots that are over the mid calf. 
2. You can wear them with pants, but then what's the point of having them sheer or patterned?
3. and in either case... you end up having bare feet, so you either go bare footed or you have to wear socks anyway. Why not just have plain old footed tights?

The only thing I can think they're really useful for is the "below mid thigh skirt with above the mid calf boots" look, and how often does one wear that, and then still, why not full-lenth tights so that you don't' have to wear socks too? 
A friend informs that they are sometimes used for roller derby, but surely that does not explain how they have become so very endemic in the stores (to the point that at some places, I can't find regular hose, just these footless things).