They go over this in chapter 1156. Rocks wants Harald to eat a certain fruit so its full potential is unlocked, which may or may not be the Shadow fruit, but he also wants Harald so the frozen giants would listen to Harald. I believe the idea is that even with the ability to resurrect the giants, they would not have any allegiance to Rocks. However, they would yield to the authority of the king of Elbaph. I don't know what the reasoning Rocks's crew has for that, but that's what they believed at the time. Also, I wasn't aware the Soul fruit could reanimate corpses like the Shadow fruit. I thought it could just bring inanimate objects to life. Maybe Rocks wanted Brook's fruit, maybe it has some special bullshit powers we don't know about.