The Simplify brush is specifically meant for use with Dyntopo to remove/add detail in the mesh.
Even when the Refine Method is set to Subdivide Edges, this brush is always able to collapse edges. This ensures that while focusing on adding detail to your sculpt, the Simplify brush can always be used to simplify and polish surfaces.
This brush has no effect if Dyntopo is disabled.
In combination with auto-smooth the brush can polish surfaces while it remeshes them. On tube-like geometry it can also shrink and dissolve volumes completely.