Why do some tights have a gusset?

Gussets are used in manufacturing of modern tights and pantyhose to add breadth at the crotch seam. As with other synthetic underwear, these gussets are often made of moisture-wicking breathable fabrics such as cotton, to keep the genital area dry and ventilated.

What is a panel in tights?

It simply means an added piece of hosiery, shaped like a V and it is inserted between the seam at the back of the tights. This is sometimes called a comfort panel, although they are rarely comfortable. This is designed to give extra width across the bottom which helps to extend the size.