Wikipedia now has a page up about them.
The fact that Captain Fitzroy carried so many (more than 20, the majority purchased at his own expense) and cherished them so well throughut the voyage is a tribute to the man's rigour and seamanship. The primary purpose of the 1831-36 voyage was not to carry Darwin to fame, but to survey the coast of South America and to 'carry a chain of chronometric measurements' around the world.
Navigation in those days was far more complex than the modern penchent for punching up the GPS, and an accurate chronometer was crucial in calculating longitude. Fitzroy has a dedicated padded cupboard set aside for his stock of chronometers and a crew member appointed to ensure that they were wound.