Interesting

When I planted the baby tree peony on its own roots, which I got form the neighbour, the peony was dormant. It lost all leaves, but there was one leaf bud, which looked alive. However, the main root looked good, much bigger than the part above the ground. It also had a little feeder root, which confused me. The peony was dormant, but the feeder root was still there.
Now, I don't quite understand why we have to plant tree peonies in the fall

They will not start growing new feeder roots until next spring. They cannot get nutrition until spring. What can they absorb from the main roots? Water only? I don't understand the cycle

Please enlighten me.