By the time a company asks for references, most candidates assume the hard part is over. The interviews went well, the offer feels close, and references seem like a formality on the way to a start date.
That assumption is often wrong, especially at the senior level. Reference checks are frequently the last real filter before an offer, and for a close call between two strong finalists, they can be the deciding factor. Yet most candidates spend far more time preparing for the interview than they do preparing their references, which is backwards given how much weight this step actually carries.