The Offset tool is used to duplicate and offset an edge loop at the intersection of boolean operators.
This way clean topology in the form of a perimeter loop is established in the periphery of the boolean operation.
It is not a general purpose offset tool.
Offset can deal with tricky geometry overlaps and significantly reduce manual cleanup work as a result of these overlaps.
It is wise to run BooleanCleanup before using the Offset tool.
Select a loop of edges or vertices. It should be a cyclic selection, otherwise non-manifold geometry will be created.
cyclic edge loop at boolean intersection or chamfer, a vert loop is fine as well