The function of the two lines is to lift the buttocks.

Extended content:

Leggings, also called underpants. Leggings are pants designed to wear short skirts and miniskirts, prevent exposure and cultivate one's morality. Due to different lengths and materials, it can be divided into many types, which can be matched with formal attire in different ways.

Because of exposure prevention and self-cultivation, not only the position of wearing is the foundation, but also the heart is relatively bottomed. Leggings are generally matched with doll shirts, long suspenders, T-shirts and jumpsuits with a slightly longer upper body. This kind of trousers is mainly suitable for female consumers, so it is favored by women.

References:

Baidu encyclopedia entry "leggings"