You need live rock to seed dry rock but not to cycle it. The bacteria you are trying to grow on dry rock will populate on its own once a food source (ammonia) is introduced to the tank and the tank cycles via that food source. Having a few lbs of LR will (1) help speed the process because some bacteria will already be present on the LR and (2) you will (assuming its good quality LR) introduce organisms that you would not have with dry rock alone. Pods (copepods, amphipods, isopods) micro stars, feather dusters, etc are just some of the beneficial things you gain by using LR (of course there are other ways to introduce those as well but LR is a great way to do it. However, LR does provide the opportunity for some bad things to be introduced (aptasia, bad isopods (not common), colonial hydroids, etc.) IMO/IME, having the diveristy that LR provides in your system is invaluable to the health and grwoth of your system and outways any potential risk for bad hitchhikers. Another way to help intoduce diverisity is by adding some macro algae and/or some sand from an established system, although you won't "seed" as much bacteria verus using a few lbs of good quality LR. Or you can buy "packages" which contain pods, stars and the like but those are typically expensive. There are a number of places to get good LR online or if you have a Petco around you they now sell Walt Smith maricultured figi rock.
