Because many geometry operations have two valid solutions. The selector permanently records which solution you intend — preventing silent wrong geometry.
Many geometry operations — intersections, tangencies, parallel offsets — have two mathematically valid solutions. Without a selector, the software must guess which one you want. Guessing produces silent wrong geometry — the most dangerous type of error in CNC programming.
XyzPilot's design principle: never guess. The direction selector forces an explicit, informed choice. That choice is recorded as part of the geometry definition and cannot change silently between sessions, software versions, or machines.