Your source stays close
Local processing means the source video can be transformed using the device itself. There is no need to send the entire clip to a Meyes-operated render queue for the core workflow.
Less waiting on an upload
Large video uploads take time and depend on connection quality. On-device processing removes that upload step, although generation speed still depends on video length, quality, and device performance.
Local does not mean limitless
Video processing is demanding. Available storage, battery, thermal conditions, format compatibility, and iPhone capability can all affect the experience. Good product design should make those constraints visible.