Drop down boxes and text fields are prefilled or preselected with reasonable default values. The default values are qualified guesses as to what the user would possibly select.
The example shown above is from the airline company called SAS. Initially, a country is selected, where after the from fields are filled with appropiate airports. Also notice how it is assumed that the user wishes to make a round trip, fly sometime in the current month, and fly alone without any children. These default choices possibly answers to the large majority of the airline’s customers.
By providing default values in often complex forms with many choices, you save the user from the hassle of selecting all the relevant choices. Filling out a long form can sometimes be enough reason for the user to go somewhere else, where the process is easier.
The default values might not be right, but at least you provided the user with an example that he can change with as much effort as he would have put in if there was no example.