When working with a team you often need to exchange assets before they're finished. We'll start by creating a prototype that can then be passed to Unity for pipeline integration and level design drafts.
Creating highly detailed models for games requires normal maps, but before those maps can be created we need to create a highly detailed sculpt with Blender's Dynamic Topology tools.
Once a high-resolution sculpt is created we need to make an optimized version of it by recreating the surface and form, minus all the detail. This is where retopology comes in.