Imagine you changed it from an array of approvals to a single one like in the original example - you'd need to make a change in every method of that type (replacing the map). That's code duplication, it's just not really obvious yet because there's only two layers to pass through. Per layer you need a constructor call (or map to copy & modify the array).

As a more obvious example, if you want to modify a.b.c.d.e (which isn't unrealistic), you'll need to call the constructors of A, B, C and D. If you don't use lenses, this is the code that will be duplicated. You can spread it between the classes or do all that in a method of A, but if you want to also modify a.b.c.d.f, you'll need to duplicate all that code (add another method to A, B, C, D that each calls the constructor).

With lenses, you define once how to access d from a and then any modification of d can happen through that. If the structure changes, you only need to do the changes once by modifying the lens.