Here's the request:
Neal Statement on Requesting President Trump’s Tax Returns. It does state a reason. It's an interesting one.
As I understand it, the President, alone among Federal officials, is exempt from conflict of interest laws. Since the president can't meaningfully audit himself, it is Congress' responsibility to see whether presidents have any significant conflicts of interest. This is a particular concern with Trump, because he has not put his businesses in escrow, as is typically done.
Here's a review of the legal situation:
Why the IRS is legally required to give Congress Trump's tax returns, but probably won't. While people disagree, there does seem to be a reasonable basis for it.