Kindly see the sentence above the red heart. You already "Raise the heat to high and sauté for about 30 seconds." Why not keep sautéing at high heat? Why "Transfer the pan to the oven"?
This second step feels dispensable! You're wasting money and energy because you have to turn on the cook top and oven!
From p. 46 in 2005 Williams-Sonoma Collection: Seafood. I don't duplicate Why roast mussels in an oven, rather than steam or stir fry in a stock pot?, because here we're not cooking exclusively mussels in an oven. We have two steps.