The Arduino Pro Mini is intended for advanced users who require flexibility, low-cost, and small size. It comes with the minimum of components (no on-board USB or pin headers) to keep the cost down. It's a good choice for a board you want to leave embedded in a project. Please note that there are two versions of the board: one that operates at 5V (like most Arduino boards), and one that operates at 3.3V. Be sure to provide the correct power and use components whose operating voltage matches that of the board.