The penalty for a lost ball or OB is stroke plus distance. That is, the player takes a one stroke penalty and returns to the spot from which the ball was struck and hits again.
So a provisional ball is a timesaver. Rather than first going forward to search, the player announces he is hitting a provisional. If the original ball is then found, the original ball is played. If the original ball really is lost or OB, the provisional is played with a one-stroke penalty.
In the rulebook, see Rule 27.

