Congressman Neal has stated that he is requesting the returns to evaluate the effectiveness of the IRS’s policy of auditing the returns of sitting presidents.

Legislative purpose established.

Now, back to the law. Please show me where it is stated that a specific legislative function is even required:

Committee on Ways and Means, Committee on Finance, and Joint Committee on Taxation
Upon written request from the chairman of the Committee on Ways and Means of the House of Representatives, the chairman of the Committee on Finance of the Senate, or the chairman of the Joint Committee on Taxation, the Secretaryshall furnish such committee with any return or return information specified in such request, except that any return orreturn information which can be associated with, or otherwise identify, directly or indirectly, a particular taxpayer shall be furnished to such committee only when sitting in closed executive session unless such taxpayer otherwise consents in writing to such disclosure.