- The search in the engine is deterministic, in the same position with the same amount of time, search will always respond with the same move. Having an opening book where the engine can chose between multiple good moves for a position introduces a bit of randomness. The engine will not always play the same game.
- The engine saves time when playing the first moves as it doesn't have to search for a move. Without an opening book the moves from the engine are also not stupid but it took time to search for them. Engines with an opening book have a slight advantage against engines without one.
The reason to program a chess engine after all is to test my ability to create one not to test my ability to copy code or rely on information in existing databases created by others.
So lets think about how to do it myself ...