When the Mormon pioneers first settled in the western United States in 1847, LDS Church leaders encouraged members to perform their fast on the first Thursday of each month, and to donate the food thus saved to their bishop.
When money, in the form of specie instead of barter, became more available in Utah Territory, members were encouraged to make their donations in cash, which could better be held until needed to purchase food.
Specifically, fast offerings are used to provide food, shelter, clothing, medical care, and other necessities for those who are in need, fulfilling the meaning conveyed in Isaiah 58:6–11, with attendant blessings to the giver and the receiver.
Tithing funds are used to build and maintain meetinghouses, temples, and educational facilities; for the general maintenance of church operations; and for costs of missionary and genealogical and family history work.
The combined contribution can include tithing, fast offerings, and other humanitarian donations, and is delivered to the leader on a "convenience" basis (i.e., there is no set time either in or outside of a formal meeting where the funds are requested).