Underwear is worn for a variety of reasons. 


They keep outer garments from being soiled by perspiration, urine, semen, pre-seminal fluid, feces, vaginal discharge, and menstrual blood.


Women's brassieres provide support for the breasts, and men's briefs serve the same function for the male genitalia. A corset may be worn as a foundation garment to provide support for the breasts and torso, as well as to alter a woman's body shape. 


For additional support and protection when playing sports, men often wear more tightly fitting underwear, including jockstraps and jockstraps with cup pocket and a protective cup. Women may wear sports bras that provide greater support, thus increasing comfort and reducing the chance of damage to the ligaments of the chest during high-impact exercises such as jogging.


In cold climates, underwear may constitute an additional layer of clothing helping to keep the wearer warm. This includes thermals for cold weather. Underwear may also be used to preserve the wearer's modesty – for instance, some women wear camisoles and slips (petticoats) under clothes that are sheer. Conversely, some types of underwear can be worn for sexual titillation, such as edible underwear or crotchless panties.


Undergarments are worn for insulation under space suits and dry suits. In the case of dry suits, the insulation value of the undergarments is selected to match the expected water temperature and the level of activity for the planned dive or water activity


Some items of clothing are designed exclusively as underwear, while others such as T-shirts and certain types of shorts are suitable both as underwear and as outer clothing. The suitability of underwear as outer clothing is, apart from the indoor or outdoor climate, largely dependent on societal norms, fashion, and the requirements of the law. If made of suitable material, some underwear can serve as nightwear or swimsuits.