To protect whistle-blowers, or other assets that require anonymity.
They're not allowed to tell us any top-secret information, but you can usually tell if something is getting hidden from the public based on their reactions. If something is getting hidden, expect Ron Wyden (or other Democrats on those Committees) to make a big, vague, stink about the matter.
In effect, the existence of those top-secret committees ensures that the redactions are fair. Unlike most committees, the law states that Democrats AND Republicans must be as close to 50/50 split on those committees as possible, due to their great importance in matters such as these.
> In effect, the existence of those top-secret committees ensures that the redactions are fair.
No, it doesn't in general, and it most obviously doesn't on matters like the Mueller Report where redactions aren't even notionally due to classification and where those conmittees don't get the unredacted versions.
But for now, I should note that Volume II of the report has very, very few redactions.
This is important because Volume I seems to implicitly exonerate Trump (at least, Muller doesn't seem to think there's anything here).
Volume II however, Obstruction of Justice, seems to be the issue that Congress should look into. Since Volume II (Obstruction of Justice) is mostly available to the public / non-secret, I'm feeling pretty confident that Congress has what it needs to act (or decide to not act). There are a few "HOM" (harm to ongoing matter) redactions, but the evidence is laid out pretty cleanly.