I know this isn't much of a leap considering that Monica Rambeau's currently going thru some kinda mutation and seemingly as a result of going into the hex and coming back out. But the reason Wanda is doing it is because of her fear of loneliness and others being afraid of her because of her powers. In Civil War she talked to Vision about feeling lonely and not wanting to cause fear in others due to her powers, in episode 6 of WandaVision she expresses the same feeling to "Pietro." My guess is that she is doing this to make people more like her. With superpowers and whatnot so they have nothing to fear. This could also be leading to her becoming the MCU's version of Magneto, with her believing she is helping people evolve and that sorta thing though she does it against their will which will be the cause for conflict.