The Congress is a co-equal branch of government (in fact, it's Article I in the Constitution) that acts as a check on Executive power:
Throughout its history, Congress has engaged in oversight—broadly defined as the review, monitoring, and supervision of the implementation of public policy—of the executive branch. The first several Congresses inaugurated such important oversight techniques as special investigations, reporting requirements, resolutions of inquiry, and use of the appropriations process to review executive activity. Contemporary developments, have only increased Congress’s capacity and capabilities to check on and check the Executive. Public laws and congressional rules have measurably enhanced Congress’s implied power under the Constitution to conduct oversight.[Emphasis in original.]It's has the right to subpoena government officials, especially in the Executive Branch, as it sees fit.
Yesterday, The House voted to authorize the Judiciary Committee the authority:
...to seek declaratory judgments and any and all ancillary relief, including injunctive relief, affirming the duty of:It's congressional oversight. It's voting to enforce a subpoena (two, actually) necessary to ensure that oversight. It's the constitutionally mandated check and balance on executive power.
(A) William P. Barr, Attorney General, to comply with the subpoena that is the subject of the resolution accompanying House Report 116-105; and
(B) Donald F. McGahn, II, former White House Counsel, to comply with the subpoena issued to him on April 22, 2019
And instead 191 House republicans voted to protect the Trump Administration.