When one is proceeding with a divorce and gathering information from the other party, the goal is to be fully informed about all of the relevant facts and circumstances of the case. Whether the case is amicable or not, both parties have a duty of candor and an obligation to provide full disclosure of information to avoid defrauding the other party and the family court. That means that one must respond to the discovery requests of the other party, but must also provide additional information (even if the other party has not requested it) if such information is relevant to a fair and equitable resolution of all the issues in the divorce.
Different cases call for different kinds of discovery. Formal discovery requests include interrogatories (i.e., written demand for answers to specific questions), requests for production of documents, and depositions (i.e., oral testimony placed on record with a court reporter, but not in a courtroom). Informal discovery is the exchange of information without those formalities, and without formal deadlines under threat of a motion to the family court for failure to comply.
Sometimes formal discovery is more than what is needed, and informal discovery will suffice; for instance, in relatively amicable divorce cases. However, sometimes formal discovery is less than what is needed, because of the concern that someone will not be deterred from defrauding the other party and the court. In that event, it may be necessary to enlist the services of a private investigator to track down missing information or expose inaccurate or incomplete records provided by the other party.
I would estimate that roughly three-quarters of cases proceed with informal discovery, avoiding extra time and expense of formal procedures. Relatively few cases (one to two percent) require the involvement of a private investigator, and the remaining minority of cases call for formal discovery.