Tommy and Billy Maximoff were in many ways the film’s most important characters. Featured as their own creations in the Disney+ series WandaVision, the twins were a manifestation of Wanda’s terrible power as the Scarlet Witch. Agatha Harkness told her it was the power of creation: the ability to create life where there was none before. At the end of the series, Wanda got things right, leaving both her sons and her husband, the vision, uncreated. At the very end, while studying the Darkhold, she heard her scream.
What made her appearance in this film more meaningful was that it revealed Wanda’s secret: she modeled her version of the cubs after those that existed in virtually every other multiverse besides hers. The multiverse, manifested in the dreams of people in other dimensions, is one of the things that drove the Scarlet Witch to grieving madness, along with the inevitably corrupting influence of the Darkhold. She dreams about the boys every night because she knows they are real, and the Darkhold has given her a map on how to steal them from another multiverse.
In addition, there is another level of meaning. Knowing that Billy and Tommy are real and not just beings created by Wanda means they’re very important to the slow-cooking revelation of the potential Young Avengers. By the time the actors playing Billy and Tommy are needed for these roles, they will be old enough to step into superhero roles.