Impure functions are often more efficient but also require that the programmer “keep track” of the state of several variables. Keeping track of this state becomes increasingly difficult as programs grow in size. By eschewing state programmers are able to conceptually scale out to solve much larger problems. The loss of performance is often negligible compared to the freedom to trust that your functions work as expected on your inputs.
Maintaining state provides efficiency at the cost of surprises. Pure functions produce no surprises and so lighten the mental load of the programmer.