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.