The tradition of a father walking the bride down the aisle is based on the thought that he is giving her away. This was done as a way for the father to give away his daughter in exchange for a dowry or price.
While the father giving away his daughter is rarely still done in exchange for a price, many people still choose to follow this tradition. However, there are many other ways a father can "give away" his daughter.
Some women choose to have both their mothers and fathers walk them down the aisle, while brides without fathers may have an uncle or grandfather walk them down the aisle. Some women prefer walking down the aisle alone as a way to give themselves away.