Despite all the power and prowess wielded by Eternals like Ikaris and Thena, it was Sersi (Gemma Chan) who was chosen to lead them all as the Prime Eternal. While yes, a large part of that decision came as a result of her compassion, drive, and natural affinity for leadership, it was also a product of her almost impossibly powerful ability to transmute matter at will.
As if the average suite of Eternal powers — the package that includes near-immortality, super strength, speed, stamina, and the rest — wasn’t enough, Sersi is a walking, talking philosopher’s stone, able to transform any substance into any other, even living material. MCU viewers have already seen wars begun over substances like vibranium and uru (from which the Infinity Gauntlet was made), and for Sersi, such things are merely byproducts of her will.
This already makes her one of the most important beings in the galaxy, more than valuable enough to launch wars for her services alone, but she can also produce gold, nuclear fuel, and whatever else she can imagine. What’s more, she surprised even herself by demonstrating the ability to turn a Deviant foe into a tree, implying Sersi can transmute life just the same as inorganic material. She’s a living factory, capable of creating food, fuel, or living weapons on a whim, which is a downright scary amount of power — and one the MCU is fortunate to have in benevolent hands.