Via Election Law Blog, this New York Times op-ed by two ex-senators gives an unconventional justification for disclosure: to help incumbents ward off anonymous attacks:

Without the transparency offered by the Disclose Act of 2012, we fear long-term consequences that will hurt our democracy profoundly. We’re already seeing too many of our former colleagues leaving public office because the partisanship has become stifling and toxic. If campaigning for office continues to be so heavily affected by anonymous out-of-district influences running negative advertising, we fear even more incumbents will decline to run and many of our most capable potential leaders will shy away from elective office.

I suppose it makes sense to argue this if you are speaking to senators’ self
Interest, but it might not look great to voters.