Water from your sump or display tank is good. It is far less harsh than newly mixed saltwater. Less chemical issues there. But water doesn't really contain much bacteria. The beneficial bacteria grows on LR
or in filters (filter balls, filter floss, filter pads, etc). Live rock is a good natural filter because it is porous and has lots of places for bacteria to live and convert ammonia. Man made filters work about the same way and there are lots of people who argue the benefit one way or the other. But water alone will not provide you with the appropriate amount of bacteria to prevent a cycle in a new tank. That's were the filter pads come in. Some people go buy them (or they give them away) from their LFS
or a friend when they need to set up a quarantine tank. My LFS
is an hour away so I keep my own. It's simple preventative practice.