I somewhat understand what a sealed indictment is; but don't understand how it can be used strategically and when it's advantageous to unseal.

"A sealed indictment an indictment that is sealed so that it stays non-public until it is unsealed. This can be done for a number of reasons. It may be unsealed, for example, once the named person is arrested."

Source: https://definitions.uslegal.com/s/sealed-indictment/