I'll say at the outset that I have no idea what's in Trump's tax returns. I mean that in both the sense that I don't know specifically what's in his, and also that I don't know what kind of information you can glean from tax returns. (I mean, I know what's in my tax return, but I'm guessing the complexity of a rich guy's returns compared to mine is like a 747 compared to a tricycle.)
I also have no idea whether the information in his returns will hurt him politically, or lead to impeachment, or what the impact will be.
The reason I want to see them is because I think transparency is really important! I thought that when Mitt Romney was reluctant to release his in 2012, and I think that now that Bernie Sanders seems to be dragging his feet (even though I suspect that Bernie's are probably simple enough that he still does them in TurboTax, if not by hand).
The fact that Trump still owns his business after becoming president is a huge issue in its own right, because it raises the possibility that he is putting his business' interests above the country's. But legally speaking, there's nothing that can be done about that. So at a minimum, Congress should have as much information as possible about those businesses and be able to assess whether he is conflicted. (I almost wrote "the public" should have that information. That is what I believe, but legally speaking, only Congress has the right to it.)
And then there's also the fact that Trump, his associates, his organization and his foundation are currently under investigation by numerous state and federal entities. Obviously, that in itself doesn't prove anything, but it does at least suggest some kind of probable cause that Congress may want to look into.
One last point: Even though the law seems to be pretty clear that Congress can get these returns, it's not like they can just show up at the IRS tomorrow and say "Gimmegimmegimme!" Mindful of the inevitable legal challenges the administration will make, the Ways and Means Committee is establishing a legal paper trail demonstrating why they need the return and why they can't get that information elsewhere. I don't know exactly what those reasons are, and it's possible they'll all be BS. But since they will be public, we will be able to judge for ourselves whether they have established a legitimate rationale for requesting the returns.