Final answer:
Merlin helps Arthur ascend to the throne due to his visionary role, recognizing Arthur's potential as a unifying and just ruler and orchestrating events to establish his kingship.
Step-by-step explanation:
The motivation for Merlin to help Arthur take on the throne in Arthurian legend stems from his role as a guiding figure and prophet. In many versions of the stories, Merlin sees in Arthur the potential for a great and benevolent ruler who can unite the fractured kingdoms of Britain. Merlin's assistance is often depicted as driven by his desire to see a stable and just rule established over a land that had been fraught with discord and turmoil. His foresight and wisdom allow him to recognize Arthur's rightful claim and potential, leading him to orchestrate the events that would elevate Arthur to the throne, including the pulling of the sword from the stone or the revelation of his royal lineage.