You can still step through functional code an examine local variables, you just have the assurance that the values of any local state are purely dependent on th

You can still step through functional code an examine local variables, you just have the assurance that the values of any local state are purely dependent on the inputs to the function and not any other external state. So it's ultimately easier to debug - any values that affect the operation of a function are explicitly set when calling the function, and you don't have to worry about figuring out ways to ensure that runtime-generate state matches whatever it was when the error originally occurred.