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 these areas dry and ventilated.
The phrase "don't bust a gusset" is an admonition to calm down; becoming so enraged and inflated that one busted a gusset would imply extreme rage and expansion beyond one's clothing capacity, e.g. like the Incredible Hulk.
Gussets are also used when making three-piece bags, for example in a pattern for a bag as a long, wide piece which connects the front piece and back piece. By becoming the sides and bottom of the bag, the gusset opens the bag up beyond what simply attaching the front to the back would do. With reference to the dimension of the gusset, the measurements of a flat bottom bag may be quoted as LxWxG.
Pillows too, are often gusseted, generally an inch or two. The side panels thicken the pillow, allowing more stuffing without bulging. Gussets are also used in other areas of manufacturing, e.g. bicycle frames employ gussets to add strength and rigidity.