Let's say Blankfein asserted his Fifth Amendment right not to answer any questions. Say Goldman was subpoenaed to provide all of Blankfein's emails. Goldman replied that, instead of complying with the subpoena, it was itself reviewing the emails in question and was considering which ones to release.
Now imagine that, nearly a year later, Goldman admitted that it had not, in fact, reviewed the emails in question, because they had been lost in a computer crash two months before it claimed to be reviewing them. Imagine Goldman also said copies of the emails were lost, because while under subpoena, it had destroyed the "backup tapes" (whatever those are) that held them and that it had also thrown away Blankfein's actual hard drive.