An offset surface is a constant distance away from the original. The offset produces an approximation of this, which is why there is a tolerance. The original surface must be smooth (no sharp corners) or the offset result is not correct.
Use offsets to "thicken" an object or to account for shrinkage in mold making.
Use offsets to intersect two (offset) surfaces to get the center curve of an implied fillet. This curve could be used for a groove milling operation to cleanly mill a fillet without actually making the fillet surface.
If an offset can be produced by an exact operation, for example, offsetting a curve and using revolution or a sweep, then use the exact operation.