This eliminates the need for the program to calculate the best lines during approximately the first ten moves of the game, where the positions are extremely open-ended and thus computationally expensive to evaluate.
On some occasions, a player might consider playing a strange move outside the opening book to force a computer to think for itself.
While this may introduce a strategic weakness, a lot of the time, playing out of the book early may end up compromising one's own pawn structure, losing a tempo or allow the opponent to develop more effectively, as chess engines have become significantly more powerful over time to think more deeply or accurately than in the past.
One of the entries without a book, DARK HORSE, defeated opponent CHAOS in part by using a nonstandard N-F3 opening.
One notable exception is the Polyglot book format which is fully documented and which is being implemented in an increasing number of programs.