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This was a surprisingly difficult question for me to answer directly. The following is the most comprehensive answer I can provide at the moment, and I will expand on it in the future for those interested.
It is an augment for a natural attraction to female legs. The skin-tight fabric that constitutes tights and pantyhose changes the appearance of the lower body significantly. The complete coverage waist to toe makes for a kind of perfection–a completion–that is not intended with stockings. While much has been discussed about the zettai ryouiki phenomenon, it is directly at odds with my own preferences. Stockings and thigh-high socks leave the thighs exposed, whereas tights cover the thighs and allow them to enjoy the same benefits as the rest of the leg. In the rare occasion that the waist and hips are exposed as well, the effect is even greater, as the border between the waistband and the exposed upper body can create an effect I would venture is similar to the zettai ryouiki. I am averse to patterns, fishnets, and damage to the fabric, as the perfection is lost with even the smallest aberration in the fabric.
The color and denier (thread density) can create a wide range of effects that other garments cannot, all of which heighten the femininity of the curves of the legs–in particular, the color gradient (preferably not skin tone), reflectivity, and the ability of the tights to cover imperfections in the skin. Examples include white tights creating an image of purity, opaque black tights showing the wearer as proper and formal, and other colors having all manner of other effects depending on color warmth, none of which are possible to the same extent with bare legs or partial coverings. In addition, I use the term “tights” as it specifically refers to opaque fabrics and deliberate colros, while “pantyhose” most often refers to sheer fabrics close to skin tone. However, black and brown pantyhose that results in dark colors dissimilar from the natural color of the legs are equivalent to tights for the above purposes.
All of this being said, it is the feet that are affected greatest by the wearing of tights and pantyhose. While the foot fetish is the most common worldwide, I have a particular fondness for feet in tights, pantyhose, and similar coverings, and do not care for bare feet except in allegorical contexts. All of the above applies doubly for the feet. It is uncommon to see characters wear tights or pantyhose without shoes, but these cases are the most ideal in showcasing the above points. Depending on the position and prominence of the feet in the image, they can have highly contrasting imagery. When visible and not the focus of the image, tights-covered feet can imply innocence, but when they are presented forward with the sole, they paint the subject as aggressive or seductive. While bare feet do the same, the covering from tights largely eliminates sanitary concerns.
In addition, it is becoming increasingly common for animated works to feature ensembles with at least one character in tights. In animated works, skin-tight coverings of different material can appear to be the same depending on their opacity and how they are depicted to reflect light. However, if it is clear that the material is not nylon or anything similar to real world materials used in the manufacturer of tights and pantyhose, the effect is greatly lessened.