If you have only dry rock it seeds the tank otherwise there isn’t any bacteria to colonize the rock. The down side is there is minimal diversity in the bacteria. A couple pieces of live rock will give you more diversity. Yes it will turn your dead rock into live rock. Nothing should ever be done just to make it faster in saltwater. It always backfires. I use liquid bacteria in my quarantine tank. I don’t keep it set up all the time only when I’m getting new fish. I soak my sponge filters in the liquid bacteria for a few hours before I set the tank up.