the answer to your question is that it is in the public interest.
You'll need to elaborate on this, the way I see it, it would be very detrimental for the Attorney General to break the law, no matter why.
The more important question is 'after two years of this being in the forefront of the news everyday, why would you NOT want the findings fully released to the public?"
I think you misinterpret deference to the judgment of the Attorney General regarding what can be lawfully made public, with some desire to keep the report secret. Everyone wants to see the whole report, we all want to know on what basis Mueller concluded as he did. If the redacted report adequately presents the bases for Mueller's conclusions, I'm fine with that, I don't suspect the redactions will be used to hide evidence of Trump's wrongdoing that Mueller should have taken into account. I know Mueller knows all of what is in his report, even whatever gets redacted, plus the details in every item of information he consulted to prepare this report. He concluded as he did and I expect he supports his conclusions.
Congress is now demanding not just the unredacted report, but every item of information, transcripts from all those interrogated suspects, probable cause hearings, depositions, agents 302s, everything Mueller saw and considered in drafting his report. Congress wants to continue investigating, it seems they think there is something Mueller overlooked, didn't properly consider, that he isn't being candid about some things they investigated. I see no basis for such views, except resistance to accept there may actually not be evidence Trump colluded with Russia to defeat Hillary.
Finally, and back on the advisability the Attorney General actually break the law, I'd note the 4 bases for Barr's redactions (forbidden by law, compromising sources and methods, matters handed off for further investigation, or harmful to the reputation of witnesses) all are legally sound and well-established practice. I'm sure even the most rabid anti-Trumper would at least agree it would be better no not make public details on matters about Trump his agents came across that were handed off for SDNY to look into. On the sources and methods, I think the sources certainly should be protected, wouldn't it be better if Trump doesn't know the names of informants on his team? On the need to protect witnesses from reputational harm, this is a matter of liability, DoJ could be liable for disclosing testimony that harmed the reputation of witnesses or suspects depending on how that information was used or not used. Finally, the secrecy of Grand Jury proceedings has a lengthy history, and I can think of a variety of reasons disclosing particularly political Grand Jury proceedings would be very detrimental.
If Mueller's report indicates he relied a lot on material within those 4 circumscribed rubrics, I'd advocate working something out, letting Congress see some of it, requiring Congress not disclose it, but I wouldn't just demand it all be made public without having a better idea of what all is there.