Based on the provided contexts, Sukuna takes over Megumi's body because Megumi has vessel potential. This is in contrast to Itadori, who was merely a cage for Sukuna. Sukuna forced Megumi to eat a finger, taking control of his body to reveal his true intentions and to potentially leave it in the future [2] [3] [4] [5].

Sukuna's possession of Megumi is also related to his obsession with messing with Itadori. By taking over Megumi, Sukuna knows he can get under Itadori's skin. Furthermore, Sukuna's actions suggest that he perceives Megumi as undervaluing himself or not reaching his full potential [6] [7].

However, the contexts do not provide explicit information on why Sukuna wants Megumi to grow stronger or what his true intentions are, aside from potentially leaving Megumi's body in the future [8].