From the Document: "Congress gathers much of the information necessary to oversee the implementation of existing laws or to evaluate whether new laws are necessary from the executive branch. While executive branch officials comply with most congressional requests for information, there are times when the executive branch chooses to resist disclosure. When Congress finds an inquiry blocked by the withholding of information by the executive branch, or where the traditional process of negotiation and accommodation is inappropriate or unavailing, a subpoena--either for testimony or documents-- may be used to compel compliance with congressional demands. The recipient of a duly issued and valid congressional subpoena has a legal obligation to comply, absent a valid and overriding privilege or other legal justification. But the subpoena is only as effective as the means by which it may be enforced. Without a process by which Congress can coerce compliance or deter non-compliance, the subpoena would be reduced to a formalized request rather than a constitutionally based demand for information."
CRS Report for Congress, R45653
Congressional Research Service: https://crsreports.congress.gov/