@rkkn and if he let Levi deal with all the problems people would say that he’s using her for her powers. Which Yuki has consistently made the point of not doing. He’s constantly wanting to be seen as her equal, especially by her.
It has been stressed that Lefi was bored with life and probably suffering from extreme depression, but Yuki has saved her from it by doing the unexpected. Like trying to ‘protect’ the strongest being we know from what she’d consider an insect dragon . Most would probably let her handle it because of her overwhelming strength, but he decided to intervene, which she thinks is endearing.
It’s also better story telling. If Yuki let Lefi do everything there’d be virtually no story. Unlike other isekai with an OP MC that’s having to learn and adjust to a new world and powers Lefi comes off as having had her powers for an incredibly long time and would just solve every problem in a second.