"We have always said that when the right opportunity presented itself, we would launch many, if not all, of our remaining ground spares, and just such an opportunity came about. Our constellation is incredibly healthy; however, the spare satellites have no utility to us on the ground. We built extra satellites as an insurance policy, and with SpaceX's stellar track record, we look forward to another successful launch, which will position us even better to replicate the longevity of our first constellation."
Source: Iridium CEO Matt Desch, explaining the selection of Iridium's next satellite launch provider, with an agreement for SpaceX to take 5 of the company's 2nd-generation satellites aboard its Falcon 9.