In an interview I don’t want to be intimidating and feigning ignorance can give the candidate opportunity to shine. If they go deep, I can keep up and keep pushing, if they veer into bullshitting I can tell and gracefully conclude without offending anyone.

As a manager, not disclosing depth lets me ask stupid questions more frequently and in more contexts (for the benefit of others, for when I forget something or don’t understand something, as a Socratic teaching method, to help set the culture of asking questions, etc)

Playing dumb is also a good way to avoid responsibility if you hate something, too, if a bit passive aggressive.